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The interpretation of tropical vegetation and crops on aerial photographs
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C o m m i s s i o n VII
I n v i t e d paper
The Interpretation of Tropical Vegetation and Crops on Aerial Photographs by R. G. M I L L E R . A t one time it seemed to me t h a t " i n t e r p r e t a t i o n " was an odd way to describe the process of e x t r a c t i n g information from aerial photographs. With more experience I have come to realise t h a t it is in fact a very apt term. The aerial photograph records its information in signs and symbols which have to be t r a n s l a t e d by each i n t e r p r e t e r into the language of his own study. Any specialist observer is likely to pick out clues ignored by others to whom their significance is not apparent, but m a n y common features will be used by interpreters w i t h different interests to produce d i f f e r e n t sets of information. The techniques of examination used by most of them will by very similar, and the special n a t u r e of the information they may individually obtain will be due to the d i f f e r e n t backgrounds of knowledge and experience possessed by each of them. The interpretation of vegetation and crops on aerial photographs of tropical areas is not in any w a y a different process from the interpretation of similar f e a t u r e s on photographs taken in other p a r t s of the world. However the tropics have their own parq ticular characteristics, and they present the air photo i n t e r p r e t e r with some novel and interesting material. Before leaving general considerations it is worth recalling t h a t the photograph may be an incomplete or even a false record of the conditions. No combination of camera and film will produce an exactly t r u t h f u l record in all conditions. Colwell has shown in various papers (most recently in [5] ) how intelligent planning can help to keep the photographic recording system as close as possible to the p a t h of rectitude. Such planning requires p ~ knowledge both of the elements to be recorded and of the response of the photographic system to them. In the tropics the required knowledge is often not available ahead of photography or there are too m a n y diverse users to j u s t i f y giving special consideration to any one aspect. F o r the most p a r t the requirement is still multi-purpose r a t h e r than specialist photography. F o r t u n a t e l y the quality of photography obtained by modern, wideangle cameras generally brings satisfaction to specialist users as well as to cartographers. It is usual to think of the tropics as a region of b r i g h t clear sunshine and to expect t h a t aerial photography should be an easy m a t t e r in these conditions. The climate in the tropics is, however, f a r from uniform, and is often extremely awkward for photographic purposes. The scarcity of landing grounds, of meteorological records and of recording stations add to the difficulties. In the truly equatorial belt (extending roughly 10 ° N o r t h and South of the equator) clear weather a t low elevations is very rare, and le Ray's description of conditions in the Ivory Coast [12] is widely applicable. When t h e r e is no cloud there is often a moist haze n e a r the ground through which it is impossible to obtain good, sharp photographs. It is sometimes very noticeable in these conditions how the quality of photography improves over hills or plateaux which rise sufficiently above the general level of the country to emerge from the low-lying mists. Many of the oceanic islands of the tropics rise abruptly from the sea to considerable heights. Their peaks are strongly cloud-forming, and the plume of cloud may persistently cover much of the land area. When the cloud is absent or of small extent, the conditions can be almost ideal and excellent photography m a y then be obtained. The plateaux of central and eastern A f r i c a and the outer zones of the continental tropics have distinct dry seasons, in the early p a r t of which there are usually clear skies and a clean atmosphere offering good conditions for aerial photography. By the middle of the dry season, however, visibility is liable to be impaired by the smoke from grass and bush fires. In West Africa the l a t t e r p a r t of the dry season is also marked by dust-
Commission VII
Tropical V~gete~tion and Crops,
Miller
233
laden winds from the Sahara, which b r i n g t h e i r effects almost to t h e coast. More unusually I have seen a similar pall reach nearly to the equator in western Uganda. In these p a r t s of the tropics conditions during the r a i n y seasons virtually preclude any extensive photography. As a result very little cover has been taken, in the growing period, and the available air photos mostly depict the vegetation and fields a t a time w h e n contrasts are probably at a minimum. A t least, however, there is some uniformity in the photography, whereas the f r a g m e n t a r y cover, which is often all t h a t is obtainable in the humid tropics, may have intervals of weeks or months between runs, and so depict the vegetation, and especially short-term crops, at many d i f f e r e n t stages of growth. With the high altitudes which the sun attains in the tropics vertical aerial photographs taken t h e r e are especially liable to th.e blemish variously known as "hot-spot", "shadow point" or "no shadow point". This occurs when a point within the photograph is directly in line with the camera-station and the sun. Objects a t and immediately around this point almost completely screen t h e i r shadows f r o m the camera. In the case of tall trees standing on the outer side of the hot-spot the camera is able to "see under" the crowns and to record the ahadow on the n e a r side of the crown image. This reversed shadow, when viewed with the normally-positioned shadow on t h e adjacent photograph, causes such ocular confusion t h a t the effects of parallax are lost and it is impossible to assess or measure the true heights of the t r e e s concerned. It is significant t h a t the best description and explanation of this phenomenon comes f r o m Sims [17] in Australia where tall, open forest is common. Such stands are r a r e in the tropics where either t h e r e is. a continuous canopy or the trees are too wide-crowned and short-boled for shadow reversal to occur. However the f r e q u e n t presence of haze over tropical ~orest brings out another effect of hot-spot. Haze does not scatter light equally in all directions and there is generally a peak of dispersion directly back along the path of the incident light [2]. As the hot-spot m a r k s the point at which the camera is directly between the subject and the object on the photograph, the back-scatter of light by haze is also at a maximum a t this point. Thus the reduction in contrast due to the loss of shadows on the photographs is aggravated by reflection f r o m the haze. Tropical air photographs often show a strong difference between average density on opposite sides, an effect which is also due to the differential scattering of light by haze. F o r t u n a t e l y electronic p r i n t i n g can now overcome most of the deleterious effects of unequal scattering by haze, but even this ingenious system cannot help with missing or reversed shadows. Probably because of the overwhelming need to obtain general purpose cover suitable for topographic m a p p i n g there has not been much research into the use of special types of photography in the tl~)pics. The Belgians have gone f u r t h e s t in investigating different combinations of film and filter, and have selected a type of "modified infra-red" as best for small-scale photography of rural areas with plenty of n a t u r a l vegetation in the Congo [16]. The only samples of this photography which I have seen covered mainly savanna areas; they were of high quality and showed more distinct tonal contrasts amongst the tree vegetation than did some older panchromatic photographs of the same area. The Belgians also make the point t h a t infra-red film is less susceptible to the effects of dry haze, and t h a t the use of this film extends the r a n g e of conditions in which photography can be undertaken - - a quality which is obviously valuable when extensive cover is required within a limited season. British efforts at testing the possibilities of i n f r a - r e d film in the tropics have not been nearly so successful. In every a t t e m p t so f a r the infra-red film has proved more liable to static electricity markings, a trouble which the Ordnance Survey in Britain also experienced when they f i r s t used infra-red film f o r photography of tidal flats. On small-scale photographs of conifer and eucalypt plantations in Kenya infra-red prints undoubtedly gave better separation of species t h a n was possible with panchromatic photographs: this advantage did not persist in really large-scale photographs when the details of crown structure r a t h e r than tone
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v a l u e s could be used a s guides to recognition. Our b i g g e s t d i s a p p o i n t m e n t w a s over t h e p h o t o g r a p h y of s t a n d s of Pinus caribaea in B r i t i s h H o n d u r a s . C o n t r a r y to t h e behaviour of conifers in Europe and N o r t h A m e r i c a t h i s P i n e is i m a g e d in a l i g h t e r tone t h a n n e i g h b o u r i n g broadleaved trees on p a n c h r o m a t i c p h o t o g r a p h s . On i n f r a - r e d p h o t o g r a p h s t h e tones of t h e broadleaved trees were lightened w h e r e a s the tone of the Pines w a s little altered, and the tonal difference between t h e two classes of tree w a s reduced r a t h e r t h a n increased. Nowhere in t h e tropics h a s it proved possible to recognise conifers f r o m other species in mixed f o r e s t by their .t~nes on small-scale p a n c h r o m a t i c film. I h a v e h u n t e d u n s u c cessfully for species of Agathis, Podocarpus and Juniperus. I c a n n o t hel0 w o n d e r i n g if t h e u s u a l tonal c o n t r a s t s between conifers a n d other t r e e s in t h e n o r t h e r n h e m i s p h e r e are m a i n l y due to the differences in the m a t u r i t y of the foliage. In the mixed f o r e s t s of the tropics conifers are associated with e v e r g r e e n trees so t h a t there is no s y s t e m a t i c difference in t h e m a t u r i t y of the foliage on a n y type of tree. I refer to t h e m a t u r i t y r a t h e r t h a n the age of the foliage for I h a v e noticed t h a t sp~"ace and larch m a y also be i n d i s t i n g u i s h a b l e by tonal c h a r a c t e r s on B r i t i s h p h o t o g r a p h s t a k e n in m i d - s u m m e r . I believe t h a t in the n a t u r a l fol~ests of New Zealand, too, the coniferous trees show no consistent tonal differences f r o m t h e m a n y other e v e r g r e e n species. The question of the scale of p h o t o g r a p h y m o s t suitable for a n y p a r t i c u l a r project is one which can only be decided a f t e r consideration of m a n y factors, not least of which is t h e economic one. A s m o s t tropical p h o t o g r a p h y so f a r h a s been t a k e n p r i m a r i l y for topographic m a p p i n g purposes, m u c h of it h a s been on scales of 1/30,000 or smaller. In s t u d y i n g f o r e s t vegetation on such p h o t o g r a p h s it is a l w a y s t e m p t i n g to t h i n k t h a t larger-scale p h o t o g r a p h s would be m u c h m o r e helpful, a n d in p a r t i c u l a r t h a t t h e y would facilitate the identification of species. In f a c t t h i s h a s v e r y seldom proved to be the case [4, 10, 18]. Most tropical forest t y p e s are so mixed t h a t it is not possible to describe or i d e n t i f y t h e m on the basis of one or even a few species. In the relatively r a r e cases of g r e g a r i o u s n e s s or single species dominance this c h a r a c t e r is reflected in the s t r u c t u r e of t h e f o r e s t a n d can u s u a l l y be recoguised even on small scale photographs. As there is little v a r i a t i o n in the canopy density of these f o r e s t s and few opportunities f o r individual h e i g h t m e a s u r e m e n t s , large-scale p h o t o g r a p h s h a v e little a d v a n t a g e for o r d i n a r y typem a p p i n g , especially since small-scale p h o t o g r a p h y n o w a d a y s is capable of such h i g h definition. I find on the c o n t r a r y t h a t there is often a d v a n t a g e in t h e concentration of chara c t e r s on small-scale p h o t o g r a p h s , a n d that, in u s i n g a stereoscope with v a r i a b l e m a g n i fication, the bulk of t h e work m a y be carried out a t low power. A f t e r all it is one of the f e a t u r e s of aerial p h o t o g r a p h s t h a t t h e y provide a bird's-eye view, which can reveal a n order or p a t t e r n t h a t is not a l w a y s a p p a r e n t a t close q u a r t e r s . There a r e obviously o t h e r l i m i t i n g factors, b u t f o r the i n t e r p r e t a t i o n of tropical vegetation it is, I believe, especially n e c e s s a r y to envisage each t y p e as a whole c o n g r e g a t i o n r a t h e r t h a n to t r e a t each one on the basis of its component species. Of course such other considerations as t h e a m o u n t of detail which can be recorded a n d the scale of m a p p i n g required to p o r t r a y it m u s t be taken into account. W h e n t h e fine p a t t e r n s of land use a n d cultivated crops are being investigated, m e d i u m or lal~escale p h o t o g r a p h y m a y be needed to show t h e essential detail a n d provide the space in which to delineate it. Optical m a g n i f i c a t i o n of the p h o t o g r a p h s u n d e r e x a m i n a t i o n is a valuable aid in i n t e r p r e t a t i o n , b u t it m a y reveal more t h a n the pen of the observer can cope with. T h e ability of the p h o t o g r a m m e t r i s t to produce m a p s a t scales considerably l a r g e r t h a n t h a t of t h e p h o t o g r a p h y p r e s e n t s other u s e r s w i t h t h e problem of u s i n g the p h o t o g r a p h s t o t h e s a m e intensity. E n l a r g e d p r i n t s are one solution, b u t I a m s u r p r i s e d t h a t there h a s not been more development of e x a m i n a t i o n stereoscopes capable of h a n d l i n g them. In the f o r e f r o n t of a n y p h o t o - i n t e r p r e t e r ' s e q u i p m e n t m u s t be his store of know-
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ledge and experience, some of which has been directly acquired but much of which may be derived from the results of earlier workers in the same or allied fields. The g r e a t e r the fund of information on which he is able to draw, the sooner he will be able to gain an u n d e r s t a n d i n g of each task he undertakes. Although rapid advances are now being made, the student of vegetation and crops in the tropics is still less well endowed in that respect than his c o u n t e r p a r t in most other p a r t s of the globe. On the other hand he is presented with more opportunities for discovery, all the more so because in many p a r t s of the tropics commanding views are denied to the ground investigator either by the mature and gentle topography or by the dense vegetation which clothes even the summits of hills. There are thus probably more instances in the tropics t h a n elsewhere of aerial photographs disclosing features in the distribution of vegetation which were previously unknown and which might othm~wise have remained in t h a t s t a t e for a long time. These discoveries may not be of such immediate i m p o l ~ n e e as t h a t of the f i r s t rocketlaunching sites a t Penemiinde, but they can b r i n g no less a thrill to the i n t e r p r e t e r and may be even more difficult to explain. The effects of h u m a n activities in the fo1Tn of cultivation p a t t e r n s may be equally inexplicable at first sight, but are usually more easily resolved t h a n purely n a t u r a l features. Between the extremes of nearly bare desert and luxuriant forest the tropics contain a very wide r a n g e of vegetation forms. These variations in structure provide a f i r s t broad basis for the classification of vegetation by photo-interpretation. A t this level there are few special problems beyond t h a t of assessing the exact limits of the more indeterminate mixtures of g r a s s and trees. In any p a r t of the world the shorter types of vegetation such as g r a s s l a n d and scrub can only be dealt with on t h e i r appearance in mass, and attention has never been focussed on single plants. I have no experience of grassland studies but have found it interesting to note how in both Africa and the Pacific the shorter and f i n e r montane grassland is clearly distinguishable from the taller and coarser grasses of the lowlands. Even more remarkable is the similarity in photo appearance of ericaceous communities in Europe and on the mountains of Africa. With the taller forms of vegetation dominated by woody plants a g r e a t e r change of attitude is required by the photo-interpreter embarking on work in the tropics. The main developments in the use of aerial photographs in f o r e s t r y took place in p a r t s of the n o r t h e r n hemisphere where the recent glacial history had produced for each locality a somewhat limited tree flora. In these conditions the f o r e s t communities were dominated by only one or two species by which they could readily be recognised and labelled. In the application of aerial photographs it was n a t u r a l and helpful to a t t e m p t to identify these key species, often as individual trees. In the troPics , however, gregariousness or single species dominance are the exceptions r a t h e r than the rule, and most of the forests, woodlands and even the savannas contain a g r e a t variety of tree species. Richards [14] has pointed out t h a t the g r e a t bulk of the tropical rain forest can only be described as a mixed association in which there axe local but a p p a r e n t l y random fluctuations in the distribution of the numerous tree species. Presumably this is the result of uniformly favourable topography and climate and long periods free from marked geological or climatic changes. These conditions, which are particularly difficult for the field worker as well as for the photo-interpreter, a p p e a r to reach a climax in the lowland Dipterocarp forests of south-east Asia with t h e i r g r e a t abundance of closely-related species. Where utilisation of such forests is fairly intensive, as in p a r t s of Malaya, there may not in f a c t be any wide variation in the total stocking of economically valuable trees, but where only a limited range of timbers can be marketed it would certainly be valuable if more help could be extracted from aerial photographs. In North Borneo a classification based on the number and size of emergent crowns is reckoned to give an indication of economic stocking [8], and it seems probable t h a t other types of correlation will be found as photographic work is intensified in other places. 1 6
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Not all tropical forests, however, are so difficult to deal with, and wherever site conditions become more restrictive on the kinds and n u m b e r s of plants which can inhabit them simpler associations and consociations may be found. Mangrove forests provide the most striking examples of these conditions with the result t h a t many species of m a n groves can be recognised directly even on small-scale air photos: their p a t t e r n of distribution is, however, usually so intricate t h a t medium or large-scale prints m a y be needed to pick them out. In the peat swamps of S a r a w a k and in British Guiana [18] there are edaphic types, often with a strong tendency to single species dominance, which can be easily detected on aerial photographs. Similarly m o n t a n e conditions give rise to vegetation types of distinct structure and composition which present no difficulty to the photo-interpreter. An outstanding instance of gregariousness is the Alpine Bamboo of the mountains of E a s t Africa, while characteristic in s t r u c t u r e although variable in composition is the moss or elfin forest of high peaks t h r o u g h o u t the tropics. In less extreme climatic condition but on rough t e r r a i n in Ceylon, de Rosayro [6] has shown t h a t aerial photographs backed by ground sampling can enable ecologically valid types to be delineated; some of these types were mixed associations and recognition was based on a complex of characters. It has been suggested by Richards in [15] t h a t little of the forests of West Africa are true-climax formations, but owe their p r e s e n t character largely to p a s t disturbance by man. As a result most of these forests a r e a patchwork consisting of a mosaic of seral stages, and it is difficult to reeognise their ecological status. Little good-quality aerial photography h a s been available for these areas, and no detailed work f r o m them has yet been published. On the other h a n d in Uganda, where many of the existing areas of closed forest are believed to have developed in recent centuries (some even in decades) the seral stages outlined, for example, by Eggeling [7] can mostly be recognised on aerial photographs by their structure. The woodlands and savannas of the tropics a r e generally less heterogeneous in composition than the closed forest associations, but they are still likely to contain m a n y species. More of their structure is visible in the aerial view, and the details of the spatial distribution of the individual plants can often be seen. A few forms, such as Palms or Pines, can be identified as single individuals (sometimes more readily by their shadows), and there are some other distinct consociations which can be recognised on aerial photographs. In general, however, t y p i n g by photo interpretation m u s t be based on the general aspect r a t h e r t h a n on the characteristics of single species, and the occurrence of any one species can only be i n f e r r e d f r o m the type in which it is likely to be found. Howard has shown how such an approach can be used for the woodlands of Tanganyika [11]. The f a c t t h a t the aerial photographs of such regions are often taken when the vegetation is dormant is possibly a more serious handicap to the "photo-interpreter t h a n has yet been realised. Among the open savanna and grassland formations of the tropics aerial photog r a p h s have revealed some interesting f e a t u r e s of vegetation distribution. The most striking of such p a t t e r n s are those in British Somaliland which Macfadyen [13] f i r s t spotted and described as "vegetation arcs" and " w a t e r lanes". Very similar but less wellformed arc p a t t e r n s have since been observed on photographs or arid country in Tanganyika, Kenya, Ethiopia and the Sudan, but I know of no record from elsewhere of the so-called " w a t e r lanes". Greenwood [9] and Worrall [19] have given f u r t h e r notes about the character of arcs, but I am doubtful if t h e i r origin has yet been fully explained. A linear a r r a n g e m e n t of termite mounds is to be seen on aerial photographs of various p a r t s of Africa. Aubreville [1] f i r s t detected f r o m the air and then studied on air photos a linear distribution of thickets on the coastal plains n e a r Accra in Ghana; he relates this p a t t e r n to the direction of the prevailing wind, bush-fires and termite mounds. I have seen m a n y other examples in savanna conditions of slight vegetation patterns, in
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Tropical Vegetation and Crops,
Miller
237
m o s t cases probably a reflection of the d r a i n a g e or t h e u n d e r l y i n g rock s t r u c t u r e . In the v e r y d i f f e r e n t conditions of the p e a t - s w a m p f o r e s t s of S a r a w a k small pit-like g a p s and others m u c h l a r g e r a n d with r a d i a t i n g a r m s a r e conspicuous on aerial photographs. A n d e r s o n of t h e S a r a w a k F o r e s t D e p a r t m e n t has, since n o t i n g t h e m on t h e p h o t o g r a p h s , been able to explain these f e a t u r e s a s due respectively to l i g h t n i n g strikes a n d windblows f r o m cyclonic storms. He h a s also spotted other a r e a s of recent a n d p a s t devastation in these Shorea albida f o r e s t s which, f r o m s c a n t y native i n f o r m a t i o n , he deduces to be the r e s u l t of t r e e s being killed by a defoliating caterpillar, a l t h o u g h no specimen of t h e insect itself h a s y e t been obtained. In still a n o t h e r c a t e g o r y a r e t h e varied b u t characteristic p a t t e r n s f o r m e d by f i r e s in open g r a s s l a n d . In all these cases t h e aerial p h o t o g r a p h s h a v e indicated f e a t u r e s either too d i f f u s e on t h e g r o u n d or too embedded in dense j u n g l e for t h e i r shape or p a t t e r n to be a p p a r e n t to t h e worker in the field. The v a r i e t y of crops cultivated in t h e tropics is a l m o s t a s g r e a t as- t h a t of t h e n a t u r a l vegetation, r a n g i n g f r o m tall coco-nut p a l m s to low g r o u n d n u t plants. There a r e cert a i n l y more t r e e a n d b u s h crops g r e w n in t h e h u m i d t r o p i c s t h a n in a n y other p a r t of t h e world, a n d t h i s widens the scope f o r identification on aerial p h o t o g r a p h s . The layout of the crops is, however, often a g r e a t e r aid to recognition t h a n the shape or even the size of the p l a n t s concerned. In m o s t regions only a f e w crops a r e cultivated in large, r e g u l a r p l a n t a t i o n s , a n d w i t h s u f f i c i e n t b a c k g r o u n d i n f o r m a t i o n these can generally be identified even on small-scale p h o t o g r a p h s . P l a n t a t i o n s of such crops a s rubber, coco-nut, tea, sisal a n d s u g a r - c a n e all h a v e d i s t i n c t c h a r a c t e r s in a n y one region: t h e s e m a y be as m u c h a reflection of t h e t y p e of site on .which the crop is grown, the method of cultivation and t h e m e a n s of h a r v e s t i n g a s of the n a t u r e of t h e p l a n t s themselves. S u g a r cane plantations, in fact, will contain fields a t m a n y d i f f e r e n t s t a g e s of g r o w t h a n d will show such a v a r i e t y of tones t h a t it m a y be d i f f i c u l t a t f i r s t to believe t h a t t h e y are all the s a m e crop. Associated f e a t u r e s such a s d r y i n g sheds for tea, r a c k s f o r sisal fibre a n d the e a r t h - w o r k s a n d polders of s w a m p rice are, of course, additional clues in the recognition of these m e r e elaborate a g r i c u l t u r a l enterprises. M a n y tropical crops a r e g r o w n in a m u c h more dispersed m a n n e r , and the photoi n t e r p r e t e r is t h e n confronted with m o r e difficult problems. Some p e r e n n i a l p l a n t s are encouraged or allowed to develop in a semi-wild s*ate.. Oil-palms a r e a n obvious example of t h i s and of a crop which m a y be screened f r o m the c a m e r a by taller vegetation. Cacao, too, is g e n e r a l l y planted in W e s t A f r i c a b e n e a t h the s h a d e of n a t u r a l f o r e s t trees. N u t m e g s and cloves, a l t h o u g h t h e y are trees of good size, u s u a l l y occur in a h a p h a z a r d m a n n e r a n d h a v e little to d i s t i n g u i s h t h e m f r o m other wild o r cultivated species. E v e n cocon u t palms, which are so distinctive in the open or in p l a n t a t i o n s , can be lost f r o m view to t h e p h o t o - i n t e r p r e t e r w h e n t h e y are s u r r o u n d e d by s e c o n d a r y g r o w t h . W i t h crops of t h i s type it m a y be possible to recognise or deduce t h e i r presence on aerial photographs, but it is well-nigh impossible to define t h e exact a r e a which t h e y occupy. Most of t h e subsistence crops of the tropics and m a n y of the cash crops are grown on small p e a s a n t holdings which a r e i r r e g u l a r in s h a p e a n d size a n d m a y contain a m i x t u r e of p e r e n n i a l a n d a n n u a l crops. In such a mosaic t h e i n t e r p r e t e r needs high q u a l i t y p h o t o g r a p h s and a good u n d e r s t a n d i n g of local a g r i c u l t u r a l practice if he is to obtain the m a x i m u m a m o u n t of i n f o r m a t i o n f r o m them. Single p l a n t s of b a n a n a s and t h e l a r g e r bush-crops g r o w n in the open are visible a t m o s t scales of photography, b u t f o r cereals a n d g r o u n d - c r o p s t h e evidence of row s p a c i n g s or t h e p a t t e r n of p l a n t i n g m o u n d s m a y h a v e to be used to i n f e r t h e crops g r o w i n g on them. C a s s a v a , f o r instance, h a s been deduced f r o m t h e d i f f e r e n t tone of p a t c h e s of subsoil t h r o w n up w h e n the roots were d u g out. Mixed cropping is a n o t h e r common f e a t u r e of p e a s a n t a g r i c u l t u r e in the tropics, which c a u s e s trouble to the p h o t o - i n t e r p r e t e r as well a s to t h e collector of a g r i c u l t u r a l statistics. I n t e r s p e r s e d w i t h the crops t h e r e will be p a t c h e s of n a t u r a l fallow, which is the principal m e a n s used by the p e a s a n t f a r m e r in t h e tropics for the res-
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toration of fertility. The earlier stages of fallow m a y still show the p a t t e r n of cultivation and so be difficult to separate f r o m the areas, actually being cropped. With short-term crops field checking, of photo-interpretation must be carried out in the same season as t h a t of photography, for fields are seldom p e r m a n e n t and t h e lay-out will be changed by the next season. In spite of these complications aerial photographs properly handled can, as shown by B r u n t ' s work in the Gambia [3], provide much information about peasa n t agriculture t h a t could hardly be obtained by any other means. Shifting agriculture is widely practised in the tropics, and in m a n y places has left its m a r k on the n a t u r a l vegetation in a m a n n e r which is clearly shown up on air photos. The present condition of m a n y tropical areas is much better appreciated a f t e r examining aerial views of shifting cultivation in progress. This system undoubtedly has its merits where the soils have little p e r m a n e n t fertility and land is plentiful. The l a t t e r condition is disappearing with the growth of populations, and before any wise steps can be taken to secure more intensive use of the land it is necessary to study how f a r the traditional f o r m s of agriculture are making use of all the resources of the environment. Aerial photographs have been of much assistance f o r t h a t purpose in Africa, and have shown, f o r example, how large a proportion of the total land area may be unusable to primitive cultivators and yet how fully they m a y exploit the types of land t h e i r tools can cope with. Man's abuse as well as his use of the land on which he lives may be only too apparent on aerial photographs to those who know the symptoms. These effects are often f i r s t shown by the condition of the vegetation - forest degraded by too frequent clearing fo~ cultivation, fallow regrowth consisting solely of woody suckers because of destruction of the top soil, concentrated grazing on termitaries as a result of overstocking, and so on. In the p a s t the worker in the tropics has often sighed for some record of w h a t conditions were like before some change of circumstances took place or some new development was initiated. Even in the short time t h a t aerial p h o t o g r a p h s have been available for the tropics they have proved their value as historical documents, and the i n t e r p r e t e r of the f u t u r e should be well able to judge the wisdom or the folly of the actions which we take today - which may hardly be a confforting thought. References.
[1]
[2] [3] [4] [5] [6]
[7] [8] [9]
A u b r e v i 11 e, A., ,,Les For~ts A l i g h t s et les Savanes ~ Termiti~res Buissonantes des Plaines de Winneba et d'Accra (Ghana)", Bois et For~ts des Tropiques, 67, 1959. B r o c k, G. C., "Physical Aspects of A i r Photography", Longmans, Green & Co. Ltd., 1952. B r u n t, M., "The Gambia Land Use & Vegetation Survey", P a p e r to the Third I n t e r - A f r i c a n Soils Conference, Dalaba, 1959.. C a h u s a c, A. B., " F o r e s t Mapping f r o m Aerial P h o t o g r a p h s in U g a n d a " , P a p e r to the 7th British Commonwealth F o r e s t r y Conference, 1957. C o I w e 1 l, R. N., "The F u t u r e for P h o t o g r a m m e t r y & Photo I n t e r p r e t a t i o n " , Photogrammetric Engineering, XXVJ5, 1959. d e R o s a y r o, R. A., "The Application of Aerial Photography to Stockmapping and Inventories on an Ecological Basis in Rain F o r e s t in Ceylon", Empire F o r e s t r y Review, 38 (2), 1959. E g g e 1 i n g, W. J., "Observations on the Ecology of Budongo Rain Forest, Uganda", Journal of Ecology, 34 (1), 1947. Francis, E. C. and W o o d , G. H. S., "Classification of Vegetation in North Borneo from Aerial Photographs", Malayan Forester, X V I I I (1), 1955. G r e e n w o o d, J. E. G. W., "The Development of Vegetation P a t t e r n s in Somaliland Protectorate", Geographical Journal, C X X I I I (4), 1957.
Com~nission VII [10] [11]
[12] [13] [14] [15] [16]
[17] [18] [19]
Tropical Vegetation and Crops,
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H e i n s d i j k, D., " F o r e s t Type Mapping with the help of Aerial Photographs in the Tropics ~', Tropical Woods, 102, 1955. H o w a r d, J. A. C., "The Classification of Woodland in Western Tanganyika for Type-Mapping from Aerial Photographs", E m p i r e F o r e s t r y Review, 38 (4), 1959. 1 e R a y, J., "Aspects et Possibilit~s de la P h o t e g r a p h i e A~rienne en Zone de For~ts Dense Africaine", Bois et For~ts des Tropiques, 61, 1958. M a c f a d y e n, W. A., "Vegetation P a t t e r n s in the semi-desert Plains of British Somaliland", Geographical Journal, CXVI (4-6) 1950. R i c h a r d s, P. W., "The Tropical Rain Forest", Cambridge University Press, 1952. R i c h a r d s, P. W., "Study of Tropical Vegetation", Unasylva, 10(4), 1956. Simonet, M. and V a n R o o s t , J., "La Prise de Vue A~rienne en Infrarouge au Congo Belge", Publication of the I n s t i t u t G~ographique du Congo Belge, 1957. S i m s, W. G., "Shadow Point", Leaflet No. 67 of the Commonwealth of Australia F o r e s t r y & Timber Bureau, 1954. S w e l 1 e n g r e b e 1, E. J. G., "On the Value of L a r g e Scale Aerial Photographs in British Guiana Forestry", E m p i r e F o r e s t r y Review 38 (1), 1959. Worrall, G. A., "The Butana Grass P a t t e r n s " , Journal of Soil Science, 10 (1), 1959.
SUMMARY. This paper reviews the conditions which generally control the procurement of aerial photographs in the tropics, refers to the effects of high sun altitude and the occurrence of haze, mentions some experiences in the use of i n f r a - r e d photography and discusses the photographic scales suitable for studies of tropical vegetation and crops. The f e a t u r e s which most distinguish tropical vegetation f r o m t h a t of other regions are described from the photo-interpreter's point of view, and stress is laid on the prevalence of mixed forest associations composed of numerous species. Attention is drawn to the discovery of some interesting vegetation p a t t e r n s in the tropics as a result of the use of aerial photographs. The photographic characters of tropical crops and some aids to their recognition are described. Finally mention is made of the use of aerial photographs in gaining an understanding of peasant forms of agriculture and in detecting the misuse of land resources. The p a p e r will be illustrated at the 1960 World Photogrammetric Congress with lantern slides prepared from aerial photographs.
R~SUM~. Cet article donne un aperqu des conditions qui, en g~n~ral, a f f e c t e n t la production des photos a~riennes dans les pays tropicaux; il relate l'influence de la haute position solaire et celle de l'atmosph~re souvent assez vaporeuse, mentionne quelques experiences acquises dans l'emploi de la photographie infra-rouge et traite des ~chelles photographiques dont on peut se servir pour l'~tude de la v~g~tation tropicale et des champs cultiv~s. Les ph~nom~nes caract~ristiques qui distinguent le plus la v~g~tation tropicale de celle d'autres r~gions, ont ~t~ d~crits du point de vue de l'interpr~te des photos a~riennes, et il fair ressortir surtout la preponderance d'une v~g~tation des for~ts mixtes, compos~s de nombreuses esl~ces. L'attention est attir~e sur la d~couverte que dans la v~g~tation tropicale il existe d'int~ressantes formations p a r t e u t identiques, d~couverte qui r~sulte de l'emploi de photos a~riennes.
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Les caract~ristiques photographiques des produits tropicaux sur pied ont 6t6 d~crits ainsi que quelques moyens pour les reconnaltre. Finalement on signale l'emploi de photos a6riennes afin d'apprendre ~ mieux conna~tre les m6thodes d'agriculture indigene et afin de d~couvrir, 6ventuellement, l'emploi d6fectueux du sol. Lors du Congr~s mondial de photogramm6trie en 1960, cet article sera illustr6 p a r des diapositives faites d'apr~s des photos a6~iennes. ZUSAMMENFASSUNG. Dieser Artikel gibt nicht nur eine ~bersicht der Bedingungen, die im allgemeinen die Produktion yon Luftbilde~a in tropischen L~indern beeintr~ichtigen, sondern es wird auch der Einflul~ yon hoher Sonnenposition und das Vorkommen von Dunst besprochen; f e r n e r werden die E r f a h r u n g e n mit Infrarotphotographie und geeignete Aufnahmemal~stiibe zum Studium tropischer Vegetation und einheimischen Ackerbaus behandelt. Die typischen Eigenschaften, durch die sich eine tropische Vegetation von der and e t e r Gebiete unterscheidet, sind vom Gesichtspunkt der Photointerpretation aus beschl~ieben, wobei besonderer Nachdruck auf das ~berwiegen von Mischwatd, der sich aus vielerlei A r t e n zusammensetzt, gelegt wird. Die Aufmerksamkeit wird auf einige interessante Vegetationserscheinungen in den Tropen gelenkt, die erst durch Luftphotographien entdeckt wurden. Die photographischen Eigenschaften des einheimischen Ackerbaus, sowie einige der Hilfsmittel, die man zu ihrer E r k e n n u n g braucht, werden beschrieben. Zum Schlu[t wird die Verwendung von Luftbildern zum besseren Verst~ndnis der einheimischen Ackerbaumethoden und zur E r m i t t l u n g yon Bodenmi~brauch erw~ihnt. Der Artikel wird zum Londoner Kongrel~ an Hand von Diapositiven aus Luftphotog r a p h i e n illustriert werden.
C o m m i s s i o n VII
I n v i t e d paper
The Interpretation of Tropical Vegetation and Crops on Aerial Photographs by R. G. M I L L E R . A t one time it seemed to me t h a t " i n t e r p r e t a t i o n " was an odd way to describe the process of e x t r a c t i n g information from aerial photographs. With more experience I have come to realise t h a t it is in fact a very apt term. The aerial photograph records its information in signs and symbols which have to be t r a n s l a t e d by each i n t e r p r e t e r into the language of his own study. Any specialist observer is likely to pick out clues ignored by others to whom their significance is not apparent, but m a n y common features will be used by interpreters w i t h different interests to produce d i f f e r e n t sets of information. The techniques of examination used by most of them will by very similar, and the special n a t u r e of the information they may individually obtain will be due to the d i f f e r e n t backgrounds of knowledge and experience possessed by each of them. The interpretation of vegetation and crops on aerial photographs of tropical areas is not in any w a y a different process from the interpretation of similar f e a t u r e s on photographs taken in other p a r t s of the world. However the tropics have their own parq ticular characteristics, and they present the air photo i n t e r p r e t e r with some novel and interesting material. Before leaving general considerations it is worth recalling t h a t the photograph may be an incomplete or even a false record of the conditions. No combination of camera and film will produce an exactly t r u t h f u l record in all conditions. Colwell has shown in various papers (most recently in [5] ) how intelligent planning can help to keep the photographic recording system as close as possible to the p a t h of rectitude. Such planning requires p ~ knowledge both of the elements to be recorded and of the response of the photographic system to them. In the tropics the required knowledge is often not available ahead of photography or there are too m a n y diverse users to j u s t i f y giving special consideration to any one aspect. F o r the most p a r t the requirement is still multi-purpose r a t h e r than specialist photography. F o r t u n a t e l y the quality of photography obtained by modern, wideangle cameras generally brings satisfaction to specialist users as well as to cartographers. It is usual to think of the tropics as a region of b r i g h t clear sunshine and to expect t h a t aerial photography should be an easy m a t t e r in these conditions. The climate in the tropics is, however, f a r from uniform, and is often extremely awkward for photographic purposes. The scarcity of landing grounds, of meteorological records and of recording stations add to the difficulties. In the truly equatorial belt (extending roughly 10 ° N o r t h and South of the equator) clear weather a t low elevations is very rare, and le Ray's description of conditions in the Ivory Coast [12] is widely applicable. When t h e r e is no cloud there is often a moist haze n e a r the ground through which it is impossible to obtain good, sharp photographs. It is sometimes very noticeable in these conditions how the quality of photography improves over hills or plateaux which rise sufficiently above the general level of the country to emerge from the low-lying mists. Many of the oceanic islands of the tropics rise abruptly from the sea to considerable heights. Their peaks are strongly cloud-forming, and the plume of cloud may persistently cover much of the land area. When the cloud is absent or of small extent, the conditions can be almost ideal and excellent photography m a y then be obtained. The plateaux of central and eastern A f r i c a and the outer zones of the continental tropics have distinct dry seasons, in the early p a r t of which there are usually clear skies and a clean atmosphere offering good conditions for aerial photography. By the middle of the dry season, however, visibility is liable to be impaired by the smoke from grass and bush fires. In West Africa the l a t t e r p a r t of the dry season is also marked by dust-
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laden winds from the Sahara, which b r i n g t h e i r effects almost to t h e coast. More unusually I have seen a similar pall reach nearly to the equator in western Uganda. In these p a r t s of the tropics conditions during the r a i n y seasons virtually preclude any extensive photography. As a result very little cover has been taken, in the growing period, and the available air photos mostly depict the vegetation and fields a t a time w h e n contrasts are probably at a minimum. A t least, however, there is some uniformity in the photography, whereas the f r a g m e n t a r y cover, which is often all t h a t is obtainable in the humid tropics, may have intervals of weeks or months between runs, and so depict the vegetation, and especially short-term crops, at many d i f f e r e n t stages of growth. With the high altitudes which the sun attains in the tropics vertical aerial photographs taken t h e r e are especially liable to th.e blemish variously known as "hot-spot", "shadow point" or "no shadow point". This occurs when a point within the photograph is directly in line with the camera-station and the sun. Objects a t and immediately around this point almost completely screen t h e i r shadows f r o m the camera. In the case of tall trees standing on the outer side of the hot-spot the camera is able to "see under" the crowns and to record the ahadow on the n e a r side of the crown image. This reversed shadow, when viewed with the normally-positioned shadow on t h e adjacent photograph, causes such ocular confusion t h a t the effects of parallax are lost and it is impossible to assess or measure the true heights of the t r e e s concerned. It is significant t h a t the best description and explanation of this phenomenon comes f r o m Sims [17] in Australia where tall, open forest is common. Such stands are r a r e in the tropics where either t h e r e is. a continuous canopy or the trees are too wide-crowned and short-boled for shadow reversal to occur. However the f r e q u e n t presence of haze over tropical ~orest brings out another effect of hot-spot. Haze does not scatter light equally in all directions and there is generally a peak of dispersion directly back along the path of the incident light [2]. As the hot-spot m a r k s the point at which the camera is directly between the subject and the object on the photograph, the back-scatter of light by haze is also at a maximum a t this point. Thus the reduction in contrast due to the loss of shadows on the photographs is aggravated by reflection f r o m the haze. Tropical air photographs often show a strong difference between average density on opposite sides, an effect which is also due to the differential scattering of light by haze. F o r t u n a t e l y electronic p r i n t i n g can now overcome most of the deleterious effects of unequal scattering by haze, but even this ingenious system cannot help with missing or reversed shadows. Probably because of the overwhelming need to obtain general purpose cover suitable for topographic m a p p i n g there has not been much research into the use of special types of photography in the tl~)pics. The Belgians have gone f u r t h e s t in investigating different combinations of film and filter, and have selected a type of "modified infra-red" as best for small-scale photography of rural areas with plenty of n a t u r a l vegetation in the Congo [16]. The only samples of this photography which I have seen covered mainly savanna areas; they were of high quality and showed more distinct tonal contrasts amongst the tree vegetation than did some older panchromatic photographs of the same area. The Belgians also make the point t h a t infra-red film is less susceptible to the effects of dry haze, and t h a t the use of this film extends the r a n g e of conditions in which photography can be undertaken - - a quality which is obviously valuable when extensive cover is required within a limited season. British efforts at testing the possibilities of i n f r a - r e d film in the tropics have not been nearly so successful. In every a t t e m p t so f a r the infra-red film has proved more liable to static electricity markings, a trouble which the Ordnance Survey in Britain also experienced when they f i r s t used infra-red film f o r photography of tidal flats. On small-scale photographs of conifer and eucalypt plantations in Kenya infra-red prints undoubtedly gave better separation of species t h a n was possible with panchromatic photographs: this advantage did not persist in really large-scale photographs when the details of crown structure r a t h e r than tone
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v a l u e s could be used a s guides to recognition. Our b i g g e s t d i s a p p o i n t m e n t w a s over t h e p h o t o g r a p h y of s t a n d s of Pinus caribaea in B r i t i s h H o n d u r a s . C o n t r a r y to t h e behaviour of conifers in Europe and N o r t h A m e r i c a t h i s P i n e is i m a g e d in a l i g h t e r tone t h a n n e i g h b o u r i n g broadleaved trees on p a n c h r o m a t i c p h o t o g r a p h s . On i n f r a - r e d p h o t o g r a p h s t h e tones of t h e broadleaved trees were lightened w h e r e a s the tone of the Pines w a s little altered, and the tonal difference between t h e two classes of tree w a s reduced r a t h e r t h a n increased. Nowhere in t h e tropics h a s it proved possible to recognise conifers f r o m other species in mixed f o r e s t by their .t~nes on small-scale p a n c h r o m a t i c film. I h a v e h u n t e d u n s u c cessfully for species of Agathis, Podocarpus and Juniperus. I c a n n o t hel0 w o n d e r i n g if t h e u s u a l tonal c o n t r a s t s between conifers a n d other t r e e s in t h e n o r t h e r n h e m i s p h e r e are m a i n l y due to the differences in the m a t u r i t y of the foliage. In the mixed f o r e s t s of the tropics conifers are associated with e v e r g r e e n trees so t h a t there is no s y s t e m a t i c difference in t h e m a t u r i t y of the foliage on a n y type of tree. I refer to t h e m a t u r i t y r a t h e r t h a n the age of the foliage for I h a v e noticed t h a t sp~"ace and larch m a y also be i n d i s t i n g u i s h a b l e by tonal c h a r a c t e r s on B r i t i s h p h o t o g r a p h s t a k e n in m i d - s u m m e r . I believe t h a t in the n a t u r a l fol~ests of New Zealand, too, the coniferous trees show no consistent tonal differences f r o m t h e m a n y other e v e r g r e e n species. The question of the scale of p h o t o g r a p h y m o s t suitable for a n y p a r t i c u l a r project is one which can only be decided a f t e r consideration of m a n y factors, not least of which is t h e economic one. A s m o s t tropical p h o t o g r a p h y so f a r h a s been t a k e n p r i m a r i l y for topographic m a p p i n g purposes, m u c h of it h a s been on scales of 1/30,000 or smaller. In s t u d y i n g f o r e s t vegetation on such p h o t o g r a p h s it is a l w a y s t e m p t i n g to t h i n k t h a t larger-scale p h o t o g r a p h s would be m u c h m o r e helpful, a n d in p a r t i c u l a r t h a t t h e y would facilitate the identification of species. In f a c t t h i s h a s v e r y seldom proved to be the case [4, 10, 18]. Most tropical forest t y p e s are so mixed t h a t it is not possible to describe or i d e n t i f y t h e m on the basis of one or even a few species. In the relatively r a r e cases of g r e g a r i o u s n e s s or single species dominance this c h a r a c t e r is reflected in the s t r u c t u r e of t h e f o r e s t a n d can u s u a l l y be recoguised even on small scale photographs. As there is little v a r i a t i o n in the canopy density of these f o r e s t s and few opportunities f o r individual h e i g h t m e a s u r e m e n t s , large-scale p h o t o g r a p h s h a v e little a d v a n t a g e for o r d i n a r y typem a p p i n g , especially since small-scale p h o t o g r a p h y n o w a d a y s is capable of such h i g h definition. I find on the c o n t r a r y t h a t there is often a d v a n t a g e in t h e concentration of chara c t e r s on small-scale p h o t o g r a p h s , a n d that, in u s i n g a stereoscope with v a r i a b l e m a g n i fication, the bulk of t h e work m a y be carried out a t low power. A f t e r all it is one of the f e a t u r e s of aerial p h o t o g r a p h s t h a t t h e y provide a bird's-eye view, which can reveal a n order or p a t t e r n t h a t is not a l w a y s a p p a r e n t a t close q u a r t e r s . There a r e obviously o t h e r l i m i t i n g factors, b u t f o r the i n t e r p r e t a t i o n of tropical vegetation it is, I believe, especially n e c e s s a r y to envisage each t y p e as a whole c o n g r e g a t i o n r a t h e r t h a n to t r e a t each one on the basis of its component species. Of course such other considerations as t h e a m o u n t of detail which can be recorded a n d the scale of m a p p i n g required to p o r t r a y it m u s t be taken into account. W h e n t h e fine p a t t e r n s of land use a n d cultivated crops are being investigated, m e d i u m or lal~escale p h o t o g r a p h y m a y be needed to show t h e essential detail a n d provide the space in which to delineate it. Optical m a g n i f i c a t i o n of the p h o t o g r a p h s u n d e r e x a m i n a t i o n is a valuable aid in i n t e r p r e t a t i o n , b u t it m a y reveal more t h a n the pen of the observer can cope with. T h e ability of the p h o t o g r a m m e t r i s t to produce m a p s a t scales considerably l a r g e r t h a n t h a t of t h e p h o t o g r a p h y p r e s e n t s other u s e r s w i t h t h e problem of u s i n g the p h o t o g r a p h s t o t h e s a m e intensity. E n l a r g e d p r i n t s are one solution, b u t I a m s u r p r i s e d t h a t there h a s not been more development of e x a m i n a t i o n stereoscopes capable of h a n d l i n g them. In the f o r e f r o n t of a n y p h o t o - i n t e r p r e t e r ' s e q u i p m e n t m u s t be his store of know-
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ledge and experience, some of which has been directly acquired but much of which may be derived from the results of earlier workers in the same or allied fields. The g r e a t e r the fund of information on which he is able to draw, the sooner he will be able to gain an u n d e r s t a n d i n g of each task he undertakes. Although rapid advances are now being made, the student of vegetation and crops in the tropics is still less well endowed in that respect than his c o u n t e r p a r t in most other p a r t s of the globe. On the other hand he is presented with more opportunities for discovery, all the more so because in many p a r t s of the tropics commanding views are denied to the ground investigator either by the mature and gentle topography or by the dense vegetation which clothes even the summits of hills. There are thus probably more instances in the tropics t h a n elsewhere of aerial photographs disclosing features in the distribution of vegetation which were previously unknown and which might othm~wise have remained in t h a t s t a t e for a long time. These discoveries may not be of such immediate i m p o l ~ n e e as t h a t of the f i r s t rocketlaunching sites a t Penemiinde, but they can b r i n g no less a thrill to the i n t e r p r e t e r and may be even more difficult to explain. The effects of h u m a n activities in the fo1Tn of cultivation p a t t e r n s may be equally inexplicable at first sight, but are usually more easily resolved t h a n purely n a t u r a l features. Between the extremes of nearly bare desert and luxuriant forest the tropics contain a very wide r a n g e of vegetation forms. These variations in structure provide a f i r s t broad basis for the classification of vegetation by photo-interpretation. A t this level there are few special problems beyond t h a t of assessing the exact limits of the more indeterminate mixtures of g r a s s and trees. In any p a r t of the world the shorter types of vegetation such as g r a s s l a n d and scrub can only be dealt with on t h e i r appearance in mass, and attention has never been focussed on single plants. I have no experience of grassland studies but have found it interesting to note how in both Africa and the Pacific the shorter and f i n e r montane grassland is clearly distinguishable from the taller and coarser grasses of the lowlands. Even more remarkable is the similarity in photo appearance of ericaceous communities in Europe and on the mountains of Africa. With the taller forms of vegetation dominated by woody plants a g r e a t e r change of attitude is required by the photo-interpreter embarking on work in the tropics. The main developments in the use of aerial photographs in f o r e s t r y took place in p a r t s of the n o r t h e r n hemisphere where the recent glacial history had produced for each locality a somewhat limited tree flora. In these conditions the f o r e s t communities were dominated by only one or two species by which they could readily be recognised and labelled. In the application of aerial photographs it was n a t u r a l and helpful to a t t e m p t to identify these key species, often as individual trees. In the troPics , however, gregariousness or single species dominance are the exceptions r a t h e r than the rule, and most of the forests, woodlands and even the savannas contain a g r e a t variety of tree species. Richards [14] has pointed out t h a t the g r e a t bulk of the tropical rain forest can only be described as a mixed association in which there axe local but a p p a r e n t l y random fluctuations in the distribution of the numerous tree species. Presumably this is the result of uniformly favourable topography and climate and long periods free from marked geological or climatic changes. These conditions, which are particularly difficult for the field worker as well as for the photo-interpreter, a p p e a r to reach a climax in the lowland Dipterocarp forests of south-east Asia with t h e i r g r e a t abundance of closely-related species. Where utilisation of such forests is fairly intensive, as in p a r t s of Malaya, there may not in f a c t be any wide variation in the total stocking of economically valuable trees, but where only a limited range of timbers can be marketed it would certainly be valuable if more help could be extracted from aerial photographs. In North Borneo a classification based on the number and size of emergent crowns is reckoned to give an indication of economic stocking [8], and it seems probable t h a t other types of correlation will be found as photographic work is intensified in other places. 1 6
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Not all tropical forests, however, are so difficult to deal with, and wherever site conditions become more restrictive on the kinds and n u m b e r s of plants which can inhabit them simpler associations and consociations may be found. Mangrove forests provide the most striking examples of these conditions with the result t h a t many species of m a n groves can be recognised directly even on small-scale air photos: their p a t t e r n of distribution is, however, usually so intricate t h a t medium or large-scale prints m a y be needed to pick them out. In the peat swamps of S a r a w a k and in British Guiana [18] there are edaphic types, often with a strong tendency to single species dominance, which can be easily detected on aerial photographs. Similarly m o n t a n e conditions give rise to vegetation types of distinct structure and composition which present no difficulty to the photo-interpreter. An outstanding instance of gregariousness is the Alpine Bamboo of the mountains of E a s t Africa, while characteristic in s t r u c t u r e although variable in composition is the moss or elfin forest of high peaks t h r o u g h o u t the tropics. In less extreme climatic condition but on rough t e r r a i n in Ceylon, de Rosayro [6] has shown t h a t aerial photographs backed by ground sampling can enable ecologically valid types to be delineated; some of these types were mixed associations and recognition was based on a complex of characters. It has been suggested by Richards in [15] t h a t little of the forests of West Africa are true-climax formations, but owe their p r e s e n t character largely to p a s t disturbance by man. As a result most of these forests a r e a patchwork consisting of a mosaic of seral stages, and it is difficult to reeognise their ecological status. Little good-quality aerial photography h a s been available for these areas, and no detailed work f r o m them has yet been published. On the other h a n d in Uganda, where many of the existing areas of closed forest are believed to have developed in recent centuries (some even in decades) the seral stages outlined, for example, by Eggeling [7] can mostly be recognised on aerial photographs by their structure. The woodlands and savannas of the tropics a r e generally less heterogeneous in composition than the closed forest associations, but they are still likely to contain m a n y species. More of their structure is visible in the aerial view, and the details of the spatial distribution of the individual plants can often be seen. A few forms, such as Palms or Pines, can be identified as single individuals (sometimes more readily by their shadows), and there are some other distinct consociations which can be recognised on aerial photographs. In general, however, t y p i n g by photo interpretation m u s t be based on the general aspect r a t h e r t h a n on the characteristics of single species, and the occurrence of any one species can only be i n f e r r e d f r o m the type in which it is likely to be found. Howard has shown how such an approach can be used for the woodlands of Tanganyika [11]. The f a c t t h a t the aerial photographs of such regions are often taken when the vegetation is dormant is possibly a more serious handicap to the "photo-interpreter t h a n has yet been realised. Among the open savanna and grassland formations of the tropics aerial photog r a p h s have revealed some interesting f e a t u r e s of vegetation distribution. The most striking of such p a t t e r n s are those in British Somaliland which Macfadyen [13] f i r s t spotted and described as "vegetation arcs" and " w a t e r lanes". Very similar but less wellformed arc p a t t e r n s have since been observed on photographs or arid country in Tanganyika, Kenya, Ethiopia and the Sudan, but I know of no record from elsewhere of the so-called " w a t e r lanes". Greenwood [9] and Worrall [19] have given f u r t h e r notes about the character of arcs, but I am doubtful if t h e i r origin has yet been fully explained. A linear a r r a n g e m e n t of termite mounds is to be seen on aerial photographs of various p a r t s of Africa. Aubreville [1] f i r s t detected f r o m the air and then studied on air photos a linear distribution of thickets on the coastal plains n e a r Accra in Ghana; he relates this p a t t e r n to the direction of the prevailing wind, bush-fires and termite mounds. I have seen m a n y other examples in savanna conditions of slight vegetation patterns, in
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m o s t cases probably a reflection of the d r a i n a g e or t h e u n d e r l y i n g rock s t r u c t u r e . In the v e r y d i f f e r e n t conditions of the p e a t - s w a m p f o r e s t s of S a r a w a k small pit-like g a p s and others m u c h l a r g e r a n d with r a d i a t i n g a r m s a r e conspicuous on aerial photographs. A n d e r s o n of t h e S a r a w a k F o r e s t D e p a r t m e n t has, since n o t i n g t h e m on t h e p h o t o g r a p h s , been able to explain these f e a t u r e s a s due respectively to l i g h t n i n g strikes a n d windblows f r o m cyclonic storms. He h a s also spotted other a r e a s of recent a n d p a s t devastation in these Shorea albida f o r e s t s which, f r o m s c a n t y native i n f o r m a t i o n , he deduces to be the r e s u l t of t r e e s being killed by a defoliating caterpillar, a l t h o u g h no specimen of t h e insect itself h a s y e t been obtained. In still a n o t h e r c a t e g o r y a r e t h e varied b u t characteristic p a t t e r n s f o r m e d by f i r e s in open g r a s s l a n d . In all these cases t h e aerial p h o t o g r a p h s h a v e indicated f e a t u r e s either too d i f f u s e on t h e g r o u n d or too embedded in dense j u n g l e for t h e i r shape or p a t t e r n to be a p p a r e n t to t h e worker in the field. The v a r i e t y of crops cultivated in t h e tropics is a l m o s t a s g r e a t as- t h a t of t h e n a t u r a l vegetation, r a n g i n g f r o m tall coco-nut p a l m s to low g r o u n d n u t plants. There a r e cert a i n l y more t r e e a n d b u s h crops g r e w n in t h e h u m i d t r o p i c s t h a n in a n y other p a r t of t h e world, a n d t h i s widens the scope f o r identification on aerial p h o t o g r a p h s . The layout of the crops is, however, often a g r e a t e r aid to recognition t h a n the shape or even the size of the p l a n t s concerned. In m o s t regions only a f e w crops a r e cultivated in large, r e g u l a r p l a n t a t i o n s , a n d w i t h s u f f i c i e n t b a c k g r o u n d i n f o r m a t i o n these can generally be identified even on small-scale p h o t o g r a p h s . P l a n t a t i o n s of such crops a s rubber, coco-nut, tea, sisal a n d s u g a r - c a n e all h a v e d i s t i n c t c h a r a c t e r s in a n y one region: t h e s e m a y be as m u c h a reflection of t h e t y p e of site on .which the crop is grown, the method of cultivation and t h e m e a n s of h a r v e s t i n g a s of the n a t u r e of t h e p l a n t s themselves. S u g a r cane plantations, in fact, will contain fields a t m a n y d i f f e r e n t s t a g e s of g r o w t h a n d will show such a v a r i e t y of tones t h a t it m a y be d i f f i c u l t a t f i r s t to believe t h a t t h e y are all the s a m e crop. Associated f e a t u r e s such a s d r y i n g sheds for tea, r a c k s f o r sisal fibre a n d the e a r t h - w o r k s a n d polders of s w a m p rice are, of course, additional clues in the recognition of these m e r e elaborate a g r i c u l t u r a l enterprises. M a n y tropical crops a r e g r o w n in a m u c h more dispersed m a n n e r , and the photoi n t e r p r e t e r is t h e n confronted with m o r e difficult problems. Some p e r e n n i a l p l a n t s are encouraged or allowed to develop in a semi-wild s*ate.. Oil-palms a r e a n obvious example of t h i s and of a crop which m a y be screened f r o m the c a m e r a by taller vegetation. Cacao, too, is g e n e r a l l y planted in W e s t A f r i c a b e n e a t h the s h a d e of n a t u r a l f o r e s t trees. N u t m e g s and cloves, a l t h o u g h t h e y are trees of good size, u s u a l l y occur in a h a p h a z a r d m a n n e r a n d h a v e little to d i s t i n g u i s h t h e m f r o m other wild o r cultivated species. E v e n cocon u t palms, which are so distinctive in the open or in p l a n t a t i o n s , can be lost f r o m view to t h e p h o t o - i n t e r p r e t e r w h e n t h e y are s u r r o u n d e d by s e c o n d a r y g r o w t h . W i t h crops of t h i s type it m a y be possible to recognise or deduce t h e i r presence on aerial photographs, but it is well-nigh impossible to define t h e exact a r e a which t h e y occupy. Most of t h e subsistence crops of the tropics and m a n y of the cash crops are grown on small p e a s a n t holdings which a r e i r r e g u l a r in s h a p e a n d size a n d m a y contain a m i x t u r e of p e r e n n i a l a n d a n n u a l crops. In such a mosaic t h e i n t e r p r e t e r needs high q u a l i t y p h o t o g r a p h s and a good u n d e r s t a n d i n g of local a g r i c u l t u r a l practice if he is to obtain the m a x i m u m a m o u n t of i n f o r m a t i o n f r o m them. Single p l a n t s of b a n a n a s and t h e l a r g e r bush-crops g r o w n in the open are visible a t m o s t scales of photography, b u t f o r cereals a n d g r o u n d - c r o p s t h e evidence of row s p a c i n g s or t h e p a t t e r n of p l a n t i n g m o u n d s m a y h a v e to be used to i n f e r t h e crops g r o w i n g on them. C a s s a v a , f o r instance, h a s been deduced f r o m t h e d i f f e r e n t tone of p a t c h e s of subsoil t h r o w n up w h e n the roots were d u g out. Mixed cropping is a n o t h e r common f e a t u r e of p e a s a n t a g r i c u l t u r e in the tropics, which c a u s e s trouble to the p h o t o - i n t e r p r e t e r as well a s to t h e collector of a g r i c u l t u r a l statistics. I n t e r s p e r s e d w i t h the crops t h e r e will be p a t c h e s of n a t u r a l fallow, which is the principal m e a n s used by the p e a s a n t f a r m e r in t h e tropics for the res-
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toration of fertility. The earlier stages of fallow m a y still show the p a t t e r n of cultivation and so be difficult to separate f r o m the areas, actually being cropped. With short-term crops field checking, of photo-interpretation must be carried out in the same season as t h a t of photography, for fields are seldom p e r m a n e n t and t h e lay-out will be changed by the next season. In spite of these complications aerial photographs properly handled can, as shown by B r u n t ' s work in the Gambia [3], provide much information about peasa n t agriculture t h a t could hardly be obtained by any other means. Shifting agriculture is widely practised in the tropics, and in m a n y places has left its m a r k on the n a t u r a l vegetation in a m a n n e r which is clearly shown up on air photos. The present condition of m a n y tropical areas is much better appreciated a f t e r examining aerial views of shifting cultivation in progress. This system undoubtedly has its merits where the soils have little p e r m a n e n t fertility and land is plentiful. The l a t t e r condition is disappearing with the growth of populations, and before any wise steps can be taken to secure more intensive use of the land it is necessary to study how f a r the traditional f o r m s of agriculture are making use of all the resources of the environment. Aerial photographs have been of much assistance f o r t h a t purpose in Africa, and have shown, f o r example, how large a proportion of the total land area may be unusable to primitive cultivators and yet how fully they m a y exploit the types of land t h e i r tools can cope with. Man's abuse as well as his use of the land on which he lives may be only too apparent on aerial photographs to those who know the symptoms. These effects are often f i r s t shown by the condition of the vegetation - forest degraded by too frequent clearing fo~ cultivation, fallow regrowth consisting solely of woody suckers because of destruction of the top soil, concentrated grazing on termitaries as a result of overstocking, and so on. In the p a s t the worker in the tropics has often sighed for some record of w h a t conditions were like before some change of circumstances took place or some new development was initiated. Even in the short time t h a t aerial p h o t o g r a p h s have been available for the tropics they have proved their value as historical documents, and the i n t e r p r e t e r of the f u t u r e should be well able to judge the wisdom or the folly of the actions which we take today - which may hardly be a confforting thought. References.
[1]
[2] [3] [4] [5] [6]
[7] [8] [9]
A u b r e v i 11 e, A., ,,Les For~ts A l i g h t s et les Savanes ~ Termiti~res Buissonantes des Plaines de Winneba et d'Accra (Ghana)", Bois et For~ts des Tropiques, 67, 1959. B r o c k, G. C., "Physical Aspects of A i r Photography", Longmans, Green & Co. Ltd., 1952. B r u n t, M., "The Gambia Land Use & Vegetation Survey", P a p e r to the Third I n t e r - A f r i c a n Soils Conference, Dalaba, 1959.. C a h u s a c, A. B., " F o r e s t Mapping f r o m Aerial P h o t o g r a p h s in U g a n d a " , P a p e r to the 7th British Commonwealth F o r e s t r y Conference, 1957. C o I w e 1 l, R. N., "The F u t u r e for P h o t o g r a m m e t r y & Photo I n t e r p r e t a t i o n " , Photogrammetric Engineering, XXVJ5, 1959. d e R o s a y r o, R. A., "The Application of Aerial Photography to Stockmapping and Inventories on an Ecological Basis in Rain F o r e s t in Ceylon", Empire F o r e s t r y Review, 38 (2), 1959. E g g e 1 i n g, W. J., "Observations on the Ecology of Budongo Rain Forest, Uganda", Journal of Ecology, 34 (1), 1947. Francis, E. C. and W o o d , G. H. S., "Classification of Vegetation in North Borneo from Aerial Photographs", Malayan Forester, X V I I I (1), 1955. G r e e n w o o d, J. E. G. W., "The Development of Vegetation P a t t e r n s in Somaliland Protectorate", Geographical Journal, C X X I I I (4), 1957.
Com~nission VII [10] [11]
[12] [13] [14] [15] [16]
[17] [18] [19]
Tropical Vegetation and Crops,
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H e i n s d i j k, D., " F o r e s t Type Mapping with the help of Aerial Photographs in the Tropics ~', Tropical Woods, 102, 1955. H o w a r d, J. A. C., "The Classification of Woodland in Western Tanganyika for Type-Mapping from Aerial Photographs", E m p i r e F o r e s t r y Review, 38 (4), 1959. 1 e R a y, J., "Aspects et Possibilit~s de la P h o t e g r a p h i e A~rienne en Zone de For~ts Dense Africaine", Bois et For~ts des Tropiques, 61, 1958. M a c f a d y e n, W. A., "Vegetation P a t t e r n s in the semi-desert Plains of British Somaliland", Geographical Journal, CXVI (4-6) 1950. R i c h a r d s, P. W., "The Tropical Rain Forest", Cambridge University Press, 1952. R i c h a r d s, P. W., "Study of Tropical Vegetation", Unasylva, 10(4), 1956. Simonet, M. and V a n R o o s t , J., "La Prise de Vue A~rienne en Infrarouge au Congo Belge", Publication of the I n s t i t u t G~ographique du Congo Belge, 1957. S i m s, W. G., "Shadow Point", Leaflet No. 67 of the Commonwealth of Australia F o r e s t r y & Timber Bureau, 1954. S w e l 1 e n g r e b e 1, E. J. G., "On the Value of L a r g e Scale Aerial Photographs in British Guiana Forestry", E m p i r e F o r e s t r y Review 38 (1), 1959. Worrall, G. A., "The Butana Grass P a t t e r n s " , Journal of Soil Science, 10 (1), 1959.
SUMMARY. This paper reviews the conditions which generally control the procurement of aerial photographs in the tropics, refers to the effects of high sun altitude and the occurrence of haze, mentions some experiences in the use of i n f r a - r e d photography and discusses the photographic scales suitable for studies of tropical vegetation and crops. The f e a t u r e s which most distinguish tropical vegetation f r o m t h a t of other regions are described from the photo-interpreter's point of view, and stress is laid on the prevalence of mixed forest associations composed of numerous species. Attention is drawn to the discovery of some interesting vegetation p a t t e r n s in the tropics as a result of the use of aerial photographs. The photographic characters of tropical crops and some aids to their recognition are described. Finally mention is made of the use of aerial photographs in gaining an understanding of peasant forms of agriculture and in detecting the misuse of land resources. The p a p e r will be illustrated at the 1960 World Photogrammetric Congress with lantern slides prepared from aerial photographs.
R~SUM~. Cet article donne un aperqu des conditions qui, en g~n~ral, a f f e c t e n t la production des photos a~riennes dans les pays tropicaux; il relate l'influence de la haute position solaire et celle de l'atmosph~re souvent assez vaporeuse, mentionne quelques experiences acquises dans l'emploi de la photographie infra-rouge et traite des ~chelles photographiques dont on peut se servir pour l'~tude de la v~g~tation tropicale et des champs cultiv~s. Les ph~nom~nes caract~ristiques qui distinguent le plus la v~g~tation tropicale de celle d'autres r~gions, ont ~t~ d~crits du point de vue de l'interpr~te des photos a~riennes, et il fair ressortir surtout la preponderance d'une v~g~tation des for~ts mixtes, compos~s de nombreuses esl~ces. L'attention est attir~e sur la d~couverte que dans la v~g~tation tropicale il existe d'int~ressantes formations p a r t e u t identiques, d~couverte qui r~sulte de l'emploi de photos a~riennes.
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Les caract~ristiques photographiques des produits tropicaux sur pied ont 6t6 d~crits ainsi que quelques moyens pour les reconnaltre. Finalement on signale l'emploi de photos a6riennes afin d'apprendre ~ mieux conna~tre les m6thodes d'agriculture indigene et afin de d~couvrir, 6ventuellement, l'emploi d6fectueux du sol. Lors du Congr~s mondial de photogramm6trie en 1960, cet article sera illustr6 p a r des diapositives faites d'apr~s des photos a6~iennes. ZUSAMMENFASSUNG. Dieser Artikel gibt nicht nur eine ~bersicht der Bedingungen, die im allgemeinen die Produktion yon Luftbilde~a in tropischen L~indern beeintr~ichtigen, sondern es wird auch der Einflul~ yon hoher Sonnenposition und das Vorkommen von Dunst besprochen; f e r n e r werden die E r f a h r u n g e n mit Infrarotphotographie und geeignete Aufnahmemal~stiibe zum Studium tropischer Vegetation und einheimischen Ackerbaus behandelt. Die typischen Eigenschaften, durch die sich eine tropische Vegetation von der and e t e r Gebiete unterscheidet, sind vom Gesichtspunkt der Photointerpretation aus beschl~ieben, wobei besonderer Nachdruck auf das ~berwiegen von Mischwatd, der sich aus vielerlei A r t e n zusammensetzt, gelegt wird. Die Aufmerksamkeit wird auf einige interessante Vegetationserscheinungen in den Tropen gelenkt, die erst durch Luftphotographien entdeckt wurden. Die photographischen Eigenschaften des einheimischen Ackerbaus, sowie einige der Hilfsmittel, die man zu ihrer E r k e n n u n g braucht, werden beschrieben. Zum Schlu[t wird die Verwendung von Luftbildern zum besseren Verst~ndnis der einheimischen Ackerbaumethoden und zur E r m i t t l u n g yon Bodenmi~brauch erw~ihnt. Der Artikel wird zum Londoner Kongrel~ an Hand von Diapositiven aus Luftphotog r a p h i e n illustriert werden.