Supplementary aerial photography with miniature cameras

Photogrammetria - Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

SUPPLEMENTARY AERIAL PHOTOGRAPHY WITH MINIATURE CAMERAS V. G. ZSILINSZKY Ontario Department o[ Lands and Forests, Toronto, Ont. (Canada) (Received August 8, 1968)

SUMMARY

This paper presents a technique of 35 mm supplementary aerial photography, developed by the Ontario Department of Lands and Forests (Canada), with discussion of technical considerations for low and high altitude operations and of cost comparisons. INTRODUCTION

Regular aerial survey photography using large cameras and film and strict controls, is costly but necessary for the production of accurate base maps and interpretations over large areas. In the years between major aerial surveys, changes occur in the landscape features of local areas, because of various processes of exploitation, construction or disaster. Some of these changes may be of vital concern to the forester, engineer, geographer, geologist and agriculturist, but the cost of making frequent, local aerial surveys, using conventional techniques in order to maintain up-to-date information, can become prohibitive. Since the newly-developed landscape features of interest generally have continuous boundaries (e.g. forest cutover), these landscape features can be delineated by using photographs with lessthan-optimum resolution. The quality of 35 mm cameras and films is now such that prints enlarged from 35 mm negatives can provide completely acceptable details of landscape changes at a small fraction of the cost of conventional photography. This paper demonstrates how 35 mm can be used to supplement regular aerial survey photography. SUPPLEMENTARY AERIAL PHOTOGRAPHY (S.A.P.)

The prime purpose of S.A.P. is map revision. This may entail the insertion of new construction features, such as roads, power lines, industrial plants and other buildings, or the mapping of modified areas, such as forest burns or cutovers, landslides, glacial movement. The S.A. photographs are 6.2 X enlargements (15 X 22.5 c m - - 6 X 9inches) of the standard 24 X 36 mm negative and are at the same scale as the conventional Photogrammetria, 25 (1969/1970) 27-38

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V. G. ZS1LINSZKY

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Fig.1. A. Contact print made from conventional aerial survey made before forest cutting. B. Enlargement from supplementary 35 mm negative showing forest cut-over.

Photogran~metria, 25 (1'469/1970) 27 38

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Fig.2. A 6.5 X enlargement compared with the original format (24 X 36 mm) and a part enlarged 30 X. Altitude of photography 2,400 m (8,000 ft.); scale of negative 1 : 100,000; Nikon F, 24 mm Auto Nikkor, 250 motor drive; Kodak Tri X 400 ASA (27 DIN), f 5.6, 1/1,000 sec. Microdol X developer.

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photographs. They are two-thirds the size of the conventional 22.5 cm (9 inch) square print and of the area they cover (Fig.lA, B). Normal overlap (60%) ensures that complete stereoscopic coverage is available and that the interpretation of these enlargements proceeds normally. The interpretability of the S.A.P., as with all other types of photography, depends on the resolving power of the lens-filmprocessing combination. It should be understood that the enlargement of a negative cannot compete in image quality with the contact print of the same negative. The enlarging process

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P L A N I M E T R I C MAP

Fig.3. Supplementary aerial photograph showing how: A. scale is determined; B. photo is processed; C. information is transferred from photo to map.

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involves another optical system and the magnification of the grain size in the emulsion. Still, with the right lens4ilm-processing combination, adequate resolution can be obtained. For example, Fig.2, which illustrates a built-up area, has been enlarged from its original size of 24 X 36 mm to 72 X 108 cm (29 X 43 inches-30 times. Cars are identifiable on this enlargement. If the average width of a car is 1.80 m (6 ft.) and an additional 1 m (3 ft.) space is allowed between each adjacent car, and if we know that the scale of the negative is approximately 1 : 100,000, the width of a car and space on the negative will measure 0.027 mm. Thus the 35 mm negative is capable of resolving at least 37 cars/ram, and this is not necessarily the limit. The scale of the S.A.P. can be determined from the existing aerial survey photos. The scale control procedure is illustrated in Fig.3A-C. Fig.3A shows the 35 mm negative projected onto the survey photo covering the same area. The enlarger is adjusted until the projected detail coincides with that in the survey photo. When the enlarger is in this position, the scale of the S.A. photo is the same as that of the survey photo (3B)and an enlargement can be made. The interpretation marked on this enlargement can then be transferred onto the map with some type of "sketchmaster" (3C). TECHNICAL CONSIDERATIONS Camera Any camera is suitable for S.A.P. if it is equipped with a good-quality lens and has reasonably fast shutter speed (preferably 1/1,000 sec). The equipment naturally will set a limit on the range of application. Motorized assemblies are available which make possible an adequate overlap even at low altitudes. The following considerations are made for the purpose of obtaining optimum results. The resolving power of the lens used is most important in this operation because the final product will be magnified considerably. The normal focal length for 35 mm cameras is 50-55 ram. These, however, provide a narrower angle of view and a larger scale than is used in conventional photography. The flight altitude must therefore be greater in order to, obtain the same coverage as In conventional photography. Wide-angle lenses are the most practical in S.A.P. because they can simulate conventional photography from the same altitude (Fig.4). Although fiducial marks are not available on 35 mm cameras, the principal point may be located at the intersection of the diagonals. The long dimension of the 35 mm negative format is either parallel (longitudinal Fig.5A) or perpendicular (transverse--Fig.5B) to the flight line. With the longitudinal position and a given overlap, the base to height ratio will be 1.5 greater than in the transverse position, thus giving a greater accuracy in height measurements; the other position provides a greater depth of fusion. However, there are other factors that will determine the choice of position: Photogrammetria, 25 (1969/!970) 27-38

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Fig.4. This diagram shows the identical focal length to picture ratio for both 24 mm and 150 mm lenses. (1) The transverse position requires the same spacing of flight lines as in conventional photography, therefore requiring less flying time than with the closer spacing required by the longitudinal position. (2) The transverse position also provides a greater leeway in navigation by maintaining lateral overlap. (3) The longitudinal position makes it possible to view the uncut film with the stereoscope, provided the film moves in the same direction as the flight. Either way requires 33% more photographs than conventional square negatives if the S.A.P. is done with a 24 mm lens from the same altitude as the conventional photography.

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A B Fig.5. The 24 X 36 mm format in both parallel (longitudinal) and perpendicu!ar (transverse) pesitions to the flight line. The type of shutter used in aerial photography must be considered with respect to: (a) adequate speed, (b) low-temperature operation. The shutter speed required in a specific situation may be calculated according to established methods (JACKSON, 1959, 1966; ONTARIO DEPT. OF LANDS AND FORESTS, 1968). However, film speed and weather permitting, l / l , 0 0 0 sec shutter speed is adequate at altitudes of 150 m (500 ft.) and up with an average ground speed of 160 km/h (100 Photo erammetria, 25 (1969/1970) 27-38

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miles/h). Cameras with a 1/250 sec maximum shutter speed may be safely used at 2,400 (8,000 ft.) above groundlevel. The danger of the shutter freezing is eliminated when shutters lubricated with silicones are used. Length of film accomodated in 35 mm cameras is generally 1.6 m (36 standard-frame exposures). Although film cartridges with 36 exposures may not be sufficient for aerial photographic purposes, cameras are available which hold extra large cassettes (e.g., 250 exposures). Others have interchangeable backs, which allow the film (either the same or different type) to be changed rapidly in flight. Filters, used as in conventional photography, may be readily attached to 35 mm cameras. Reference material on filters is available (KODAK, 1963; AVERY, 1968). Installation of the camera in the aircraft

The aircraft to be used for this type of aerial photography must be able to accomodate the camera for vertical optical axis. If the camera is remote-controlled, the mount may be either internal or external. The most convenient arrangement, however, is a hole in the floor, which provides easy access to the camera and thus enables the operation to be completely controlled. Many small aeroplanes have a cargo-hatch in which a custom-designed camera mount can be installed efficiently. Aircraft with no cargo-hatch can be equipped with this feature at a relatively low cost. However, any modification such as this must meet the specifications of the local transport authority. The camera mount over the cargo-hatch should have the following features (Fig.6): (1) A base-plate which is transparent to provide visibility for exposure in-

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V. G. ZSILINSZKY

terval determination and for some visual control by the camera operator. The exposure interval determination is made by using a guideline marked in the transparent base-plate and a viewing stick attached thereto. Details of construction and use of this device will follow. (2) An outer circle, which permits the rotation of the whole camera mount for crab compensation. There may be several mechanical solutions for the rotation of the camera mount. Fig.6 shows a sliding metal ring which is restrained by the slack turn-buttons of the cargo-hatch. (3) A camera stand, so constructed that it can conveniently lock or release the camera. (4) Three levelling screws and a level bubble to level the camera in flight. (5) Shock absorbers to minimize the transmitted vibration of the aircraft. These are made of soft rubber, which is preferable to airfoam because it responds faster. (6) A port cover that can be closed while the camera is not in position. The camera position should be within the shell of the aircraft, but close enough to it to avoid obstructing the wide-angle field of view. The camera should not be mounted while the aircraft is taking off, landing or accelerating. During these periods a spray of fuel, dust or water may foul the lens or filter or shock may be transmitted to the camera. Aircraft and camera operation Supplementary photography may well be conducted by using small aircraft. This introduces certain restrictions, particularly in regard to navigational aids and maximum altitude. But such aircraft may be more likely available and less costly. Since S.A.P. is really spot photography rather than extensive coverage by many parallel flight lines, the direction of flight is specified by the nature of the individual assignment.

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In sequence photography, the specified forward overlap must be controlled. In order to do this in S.A.P., a guideline and viewing stick can be designed for a given lens and picture format (Fig.7A). If l represents the picture length (longitudinal: 36 mm or transverse: 24 mm), f is the focal length of the lens, L is the projected picture length on the transparent plate in the direction of flight and F is the viewing height over the guideline, then L = lxF/[, and since the net gain is 40% in a normal (60%) overlap photography, I = LxO.4. The M1, M2, lines will set the range I; the time required for an image, viewed from point 0, to traverse this range is the exposure interval (Fig.7B). The time duration of a moving image from M 1 to M 2 in Fig.7B is determined by a stopwatch and then recorded in the process of sequence photography, either manually with a stop-watch or automatically with an intervalometer. Sixty percent forward overlap is adequate in S.A.P. By using a 24 mm lens, the photo sequence as illustrated in Fig.5A (longitudinal) will give the same 2.4 ;< stereo exaggeration as provided by the 15 cm (6 inch) lens used for regular aerial photography. If the arrangement in Fig.5B (transverse) is applied, the shorter airbase will give a reduced stereo effect. By using JACKSON'S (1959) formula, where b is the photo-base in the scale of the negative and [ is the focal length, the exaggeration will be: E=

4b 4 X 9.6 -------1.6 f 24

Lateral overlap, if applicable, is well secured with 30% coverage. A ltimeter

In S.A.P., adequate determination of the flight altitude is provided by the altimeter of the aircraft. This is important only to ensure that the longer side of the 24 X 36 mm negative includes nearly the full ground distance covered by either side of the 22.5 cm (9 inch) square survey photo. Scale control has been discussed above. Negative material, exposure and processing

Film is chosen according to the type of information required. When using 35 mm cameras, one can afford any type of film. Generally, however, panchromatic film is popular because of its wide range of speed, well controlled resolution, and simple processing technique. For practical reasons it is preferable to use a fast film, provided grain size and resolution are satisfactory, so that safe shutter speeds are still available in poor light conditions. Fig.2 was taken on 400 ASA (27 DIN) film. Exposure is determined in the usual manner (ONTARIO DEPT. OF LANDS AND FORESTS, 1968). The combination of exposure and developing procedure influences the image quality of the negative film. The outcome is accidental if there is no control, but the image will be within the average quality range if the manufacturer's Photogrammetria, 25 (1969/1970) 27-38

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v.G. ZSILINSZKY

instructions are followed regarding film speed and the use of chemicals. In order to control the quality of black and photography without getting too involved, a sensitometric step-wedge m a y be printed on the film for each tank load. This done, the characteristic curve of the processed film can be plotted to check the " g a m m a " obtained. A preliminary experiment is necessary to determine what exposure and developing combination will result in the specified g a m m a value for a certain type of film and developer. It is desirable that low-contrast scenes (wooded areas) be developed to a g a m m a of 1.0, while scenes of greater contrast will require a lower g a m m a (ONTARIO DEPT. OF LANDS AND FORESTS, 1968). Film annotation

The processed film must be annotated so that any particular negative can be easily identified. The manufacturer numbers each roll of 35 m m film on one side. O n long rolls this can be used in combination with other identifying figures. For example, on 10 m rolls of Kodak film (250 exposures), a series from 1 to 44 is repeated almost six times. A subnumber for each set of 1 to 44 will identify the particular negative, such as 41/3; another figure could then be used to indicate the roll number, e.g. 4 1 / 3 - 2 . The annotated film can then be enlarged so that the figures will appear permanently on the side of the enlargement (Fig. I B). Flight report

The flight report is a permanent d o c u m e n t that should include all basic information about every photographic assignment. Its format may vary and it can be designed to suit individual taste.

TABLE I COST COMPARISON OF CONVENTIONAL AND 3 5

mm

SYSTEMS

Conventional 22.5 cm square

Miniat,re 35 m m

Ratio of com'e,tional/35 mm

$ 20,000 $ 1,000 $ 340

$ 1,400 $ 30 $ 100

14 33 3.4

$ 120 18 liters

$ 5 1 liter

24 18

2/3 2/3

l 1

3 2/3

1

3

1

0.66

Investment

Camera assembly Film processing equipment Positive printing equipment Stock items

Film (considers larger number of 35 mm negatives) Film processing chemicals Light sensitive paper (considers smaller area coverage) Positive processing chemicals

0.66 0.66

Manpower

Film processing Positive processing

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COST COMPARISON

High-quality 35 mm equipment is far cheaper than large survey cameras. Small aircraft are less costly to operate than the sophisticated survey aircraft. Furthermore, relatively inexperienced persons can be taught to handle the 35 mm equipment quickly. The cost comparison of Table 1 is only approximate, because different installations or jobs will introduce different values. LARGE-SCA LE PHOTOGRAPHY

Stress has been put on supplementary aerial photography, using the 35 mm equipment; however, large-scale, low-altitude photography can also be handled in many cases by 35 mm equipment (Fig.8).

Fig.8. A contact print of a 35 m m stereo strip. Altitude of photography 150 m (500 ft); scale 1: 2,800. Nikon F, 55 mm Auto-Micro Nikkor, 250 motor drive, 4 frames/sec; Kodak Tri X, 400 ASA (27 DIN), f 5.6, 1/1,000 sec. Microdol X developer.

During the past few years, the literature has been enriched by several studies of large-scale photography taken with 7 0 r a m cameras (ALDRICH et al., 1959; KIPPEN and SAYN-WITTCENSTEIN, 1964; LYONS, 1967; SAYN-WITTGENSTEIN and ALDRED, 1967; AVERY, 1968). These studies provide solutions to many technical problems, such as scale determination and the use of the final product for forest sampling. Many principles as laid down for the motorized 70 mm cameras also apply here. High-precision scale determination is essential for large-scale photography. Radar and laser altimeters have been recently developed for this purpose (JENSEN and RUDDOCK, 1965; DEPARTMENT OF FORESTRY AND RURAL DEVELOPMENT, 1967; WESTBY, 1967). Large-scale photography requires flying at a relatively low altitude. Normal or telescopic lenses are most useful for this purpose because, as mentioned earlier, they provide a narrower angular view than a wide-angle lens, and thus a larger scale from the same altitude. The image at low altitudes moves so fast that accurate timing of the interval is difficult. This situation may be aided by reference charts where precalculated figures are readily available. Also, the fast repetition of exposure and rewinding to ensure satisfactory stereo coverage requires a fast motor-drive system. Finally, the Photogrammetria, 25 (1969/1970) 27 38

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rapid image motion must be compensated for by adequate speed of both film and shutter. CONCLUSION

It must be understood that the recommended use of 35 m m air photography is not intended to be an attack on conventional air photography, but rather considered as an adjunct to it. In order to make use of the possible advantages, one must be prepared to accept the limitations of the technique in alle aspects of photogrammetry and p h o t o i n t e r p r e t a t i o n . If the m a t t e r of quality is faced up to, it will indeed p r o v e a stimulating challenge. T h e 35 m m p h o t o g r a p h y is not a new technique. But d o m a p p e r s a n d p h o t o interpreters fully realize its potential? I think not. T o d a y , with r a p i d l y a d v a n c i n g technology, even this m o d e s t - s i z e d negative deserves increased attention a n d trust. ACKNOWLEDGEMENTS I wish to t h a n k Prof. K. B. Jackson, Spar A e r o s p a c e P r o d u c t s Ltd., and Messrs. G. P i e r p o i n t and W. E. Jenns, O n t a r i o D e p a r t m e n t of L a n d s and F o r e s t s for their constructive criticism and editorial advice. T h e valued assistance of Mr. J. R. G. Smyth a n d others with the O n t a r i o D e p a r t m e n t of L a n d s and F o r e s t s is also much appreciated. REFERENCES ALDRICH, R. C., BAILEY, W. F. and HELLER, R. C., 1959. Large-scale 70 mm colour photography techniques and equipment and their application to a forest sampling problem. Photogrammetric Eng., 25(5): 747-754. AVERY, T. E., 1968. Interpretation of Aerial Photographs. Burgess, Minneapolis, Minn., 324 pp. DEPARTEMENT OF FORESTRYand RURALDEVELOPMENT, 1967. Radar altimeter aids-forest inventory. Dep. Forestry, Rural Develop., Can., Res. News, 10 (3): 8-10. JACKSON, K. B., 1959. Factors affecting the interpretability of air photos. Can. Surveyor, 14 (10): 454-464. JACKSON, K. B., 1966. Image motion nomograph. Photogrammetric Eng., 32 (2) 327-329. JENSEN, H. and RUDDOCK, K. A., 1968. Applications o/ a Laser Profiler to Photogrammetric Problems. Aero Service, Litton Industries, Philadelphia, Pa., 20 pp. K1PPEN, F. W. and SAYN-WITTGENSTEIN,L,, 1964. Tree measurements on large-scale, vertical, 70 mm air photographs. Can. Dept. Forestry Publ., 1053:16 pp. KODAK, 1963. Kodak Filters and Polar-Screens. Kodak Publication No. B-l, Rochester, N.Y., 49 pp. LYoNs, E. H., 1967. Forest sampling with 70 mm fixed airbase photography from helicopters, Photogrammetria, 22: 213-231. ONTARIO DEPARTMENT OF LANDS AND FORESTS, 1968. Specifications for Aerial Photography. Ontario Department of Lands and Forests, Toronto, Ont., 24 pp. SAYN-WIXTGENSTE~, L. and ALDRED, A. H., 1967. Tree volumes from large-scale photos. Photogrammetric Eng., 33(1): 69-73. WESTBY, R. L., 1967. A Radar Altimeter Jor Photogrammetric Survey o] Forests. National Research Council, Ottawa, 10 pp.

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