Remote sensing for planning and managing the great barrier reef of Australia

Photogrammetria, 40 (1985) 21--42 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

21

REMOTE SENSING FOR PLANNING AND MANAGING THE GREAT

BARRIER R E E F O F A U S T R A L I A

DAVID L.B. JUPP', KEVIN K. MAYO ~, DEBORAH A. KUCHLER 2, DAN VAN R. CLAASEN 3, RICHARD A. KENCHINGTON 3 and PETER R. GUERIN 4 ' CSIRO Division of Water and Land Resources, G.P.O. Box 1666, Canberra City, A C T 2601, Australia 2James Cook University o f North Queensland, Townsville, Australia 3 Great Barrier R e e f Marine Park Authority, Townsville, Australia 4Australian Survey Office, Canberra, Australia

(Received January 16, 1985; accepted for publication March 15, 1985)

ABSTRACT Jupp, D.L.B., Mayo, K.K., Kuchler, D.A., Claasen, D.V.R., Kenchington, R.A. and Guerin, P.R., 1985. Remote sensing for planning and managing the Great Barrier Reef of Australia. Photogrammetria, 40: 21--42. The CSIRO Division of Water and Land Resources has cooperated with the Great Barrier Reef Marine Park Authority (GBRMPA) and the Australian Survey Office (ASO) to establish comprehensive remote sensing based reef mapping procedures. The research established the potential benefits of remotely sensed data to planning and managing the GBR Marine Park as well as methods of analysis specific to the applied problems of reef top mapping. The ASO has completed a full scale mapping program (involving some 24 Landsat scenes) based on this research and development in the GBR Region. The products consist of rectified Landsat imagery at 1:250 000 and 1:100 000 scales and standard thematic products which indicate approximate bathymetry, reef geomorphological and cover zones and reef environmental parameters such as exposure to weather.

1. INTRODUCTION The Great Barrier Reef (GBR) of Australia stretches about 1900 km along t h e t r o p i c a l c o a s t o f Q u e e n s l a n d f r o m T o r t e s S t r a i t in t h e n o r t h t o its s o u t h e r n m o s t p o i n t a t L a d y E l l i o t t R e e f , s o m e 90 k m o f f s h o r e f r o m G l a d s t o n e ( Fig. 1). In 1 9 7 6 , t h e A u s t r a l i a n G o v e r n m e n t e s t a b l i s h e d t h e G r e a t B a r r i e r R e e f M a r i n e P a r k A u t h o r i t y ( G B R M P A ) w h o s e t a s k it is t o p r o g r e s s i v e l y d e c l a r e a n d p l a n areas ( c a l l e d S e c t i o n s ) o f t h e G B R M a r i n e P a r k so t h a t t h e c o m p e t i n g i n t e r e s t s o f c o n s e r v a t i o n a n d h u m a n use {such as f i s h i n g a n d t o u r i s m ) are b a l a n c e d t h r o u g h a P a r k z o n i n g a n d m a n a g e m e n t s y s t e m . T h e G B R M a r i n e P a r k , b e c a u s e o f its e x t e n t , p o s e s m a n y p r o b l e m s f o r a m a n a g e m e n t b o d y like G B R M P A w h e n t h e basic d a t a f o r i n v e n t o r y ,

0031-8663/85/$03.30

© 1985 Elsevier Science Publishers B.V.

22

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23 planning or man agem e nt need to be collected. The m ost obvious problem is scale, since the GBR covers an area of roughly 345 000 km 2 . This area is covered by tw o CZCS images, a b o u t 24 Landsat scenes and thousands of aerial photographs. An accurate map base for the entire GBR region, either for navigation or referencing, does n o t y e t exist. A n o t h e r significant problem is the logistic difficulty and expense of field visits. The weather effectively restricts productive field work to the early summer (August to November), when the strong southeast winds abate and the water is clearest. Only a small area of the GBR can be covered each season -- a fact which has caused m a n y problems for Australian Survey Office (ASO) survey teams establishing field control. In this situation, r e m o t e sensing at air and spacecraft altitudes becomes potentially an i m p o r t a n t means for surveying and monitoring the large areas of the GBR. In particular, the Landsat series of satellites offers a means for surveying, at a useful scale, m a n y of the very large, relatively u n k n o w n and inaccessible areas of the GBR at relatively low cost. The CSIRO Division of Water and Land Resources has cooperat ed with the GBRMPA and the ASO in a project which used Landsat MSS data to create an initial map base (both cartographic and thematic) for use by the GBRMPA in its task of planning Sections of the GBR Marine Park and by scientists engaged in reef survey. This paper describes the products making up that base and the reports arising f r om that project. 2. THE PHYSICAL ENVIRONMENT FOR REMOTE SENSING The spectral variation f o u n d in imagery of reef areas can be explained by the physical processes which generate the signals recorded by the satellite's sensors within the physical environment of the target reefs. Briefly, the radiance recorded at the satellite may be written as: Lsat = Latm + T a (Lsurf + Lwat) where: Lsat Lat m Ta Lsurf Lwa t

= = = = =

radiance r e c or ded at the satellite; radiance from the atmosphere; atmospheric transmission; radiance f r om the water surface; and radiance f r om beneath the water surface.

Of these terms, the radiation emerging f rom the water column (Lwat) is the objective o f r e m o t e sensing while Latm, Ta and Lsurf, which are functions of the state o f the highly variable environment of the reefs at the time of the overpass, generally decrease the effectiveness of r e m o t e sensing. The main surface effects are loss of useful energy by reflection, and the addition o f u n w a n t e d energy (Lsurf) to the r e m o t e l y sensed signal by sunglint. Direct sunlight penetrates the water column better than skylight

24

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Fig. 2. Rectified Landsat colour composite of Sudbury Reef, Great Barrier Reef, Australia.

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26 and wind increases glint. It follows that the Lwat term in Lsat is maximized, and conditions for remote sensing of the GBR are best, when the wind speed is low, the sky is clear and the sun is high in the sky. Also, since the contribution to Lwat by light reflected from reef covers decreases rapidly with increasing water depth, it is most desirable for the tide to be low. The co-occurrence of Landsat overpasses with high sun (summer), clear skies, low wind speeds and low tides is considered elsewhere (Jupp et al., 1981a; Jupp and Mayo, 1982). For this paper it is sufficient to note that the conjunction of the four desirable conditions is rare. In particular, the summer has highest cloud cover so that many cloud free scenes have low (winter) sun angles with correspondingly low depth of penetration of light and higher surface reflection. The interaction between the sun-synchronous orbit of the satellite and the tide cycle is also significant since the time of day of the overpass ensures that the lowest tide is a neap and not a spring low water. Accordingly, reefs in Landsat images of the GBR are generally submerged, and the depth of water is a basic physical and spectral gradient in the Lwat term. The physical environment of the GBR and the obvious problems of remote sensing through atmosphere and water therefore combine to make the task difficult. The same circumstances, however, cause difficulties for all reef survey, particularly for the continuing program of low-level colour aerial photographic coverage of the GBR. 3. CARTOGRAPHICMODEL An immediate benefit from satellite remote sensing of the GBR region has come from the large area synoptic view and relatively low geometric distortion in Landsat imagery. It has only been since the Landsat series that the relati'..~e positions, scale and orientation of many reefs have been adequately mapped. As part of the present exercise, Landsat imagery has been rectified and registered to the Australian Map Grid using a satellite model and the available control from surveyed reefs and topographic maps of the coast. This control is very good on individual surveyed reefs but overall the poor spread of control made it necessary to stabilize the model. The resulting imagery was produced to satisfy 1:250 000 map accuracy standards (Jupp et al., 1982) and plotted on an Applicon Inkjet plotter located at CSIRO Division of Water and Land Resources. The first of the standard products for the GBR region is therefore Landsat colour composite imagery annotated (Fig. 2) with a grid and graticule and plotted at 1:100 000 and 1:250 000 scales. These data have provided an interim mapping base for the GBR while traditional surveys proceed. Because of the logistic difficulty of field survey the possibility of using Landsat TM, SPOT and Large F o r m a t Camera (LFC) imagery to refine this base is under active consideration.

27 4. STANDARD THEMATIC MAPS For the GBRMPA, the map base is a referencing system for information about reefs and their relationships with one another which is useful for planning and managing the GBR Marine Park. Landsat data have been shown to provide not only a referencing system but also a means for providing useful thematic information about reef cover and reef morphology. Besides rectified colour composites like that shown in Fig. 2, a set of standard thematic products are being produced for the GBR region. These are: (a) Depth of Penetration images; (b) Exposure to Weather images; and (c) Classification images. Depth of Penetration images (Fig. 3} are approximate bathymetric images but are n o t referred to as such. In many areas only one image may be available, and it is difficult to separate water mass differences, turbidity and depth in a single image. The images are therefore labelled to refer to their physical basis, which is the depth of penetration of individual bands. This may be modified by either the sea floor or water properties. In the GBR region some work has been done with CZCS imagery (Claasen et al., 1984) which shows how the water masses of the GBR and ocean may separate into Oceanic water, Shelf-break water, GBR Lagoon water and one or two near shore water types. Most reefs occur in the well mixed Lagoon water which has low inorganic suspended solids concentration {SSC) but carries organic solids. The coastal waters have high inorganic SSC and are n o t well mixed. In this situation Depth of Penetration provides useful and consistent bathymetric information within the Lagoon but n o t in the near-shore area. Reefs on the oceanic boundary may also need a separate interpretation. Exposure images (Fig. 4) have been devised by using Landsat band 4 as an approximate sea floor elevation model and finding local slope and aspect by numerical differentiation. They provide a dramatic enhancement of reef morphology in the same way that relief shading helps in geomorphological mapping on land. As described later they are also valuable for biological interpretation of reefs. Classification images (Fig. 5) use the Landsat data to indicate a combination of depth and the composition of the reef cover. At the most general level, composition may be reduced to cover by coral, coralline algae and sand. On the GBR the pure carbonate sand and the darker coral/algal components have such high contrast that a satisfactory level of mapping accuracy is possible. However, it is clear that variations in depth and substrate type are very difficult to separate. The products have therefore been produced as invitations to reef scientists to interpret them into useful information using computer based or manual overlay. These standard products which were developed by CSIRO and are being produced by the ASO have been described in a number of reports arising from this project {Jupp et al., 1984a, b) and are based firmly on the phys-

28

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30 ical interactions underlying the signal sensed by the satellite. In particular they may be described in terms of a water radiance model. 5. DEPTH OF PENETRATION The radiance recorded at the water surface (that is, Lwat) can be reasonably well described by the following equation (Doak et al., 1980): L i = Lwi + (Lbi - Lwi)e -2kiz

(1)

where: L i Lwi Lbi K i

z

radiance in band i; deep water radiance in band i; = (wet) b o t t o m radiance in band i; = attenuation coefficient in band i; and = water depth. =

=

For constant water colour, turbidity and b o t t o m type the Landsat band radiances vary directly with depth of water down to the limit of depth penetration for each band. The depth of penetration of a Landsat band is that depth beyond which returning signals are not distinguishable above physical and sensor noise. This level is a function of constant factors such as sensor resolution and sensitivity as well as variable factors such as water depth and atmospheric haze. In clear and calm oceanic water with clear sky these depths are approximately 15 to 20 m for band 4, 4 to 5 m for band 5 and 50 cm for band 6. Band 7 is fully absorbed. These penetrations may be inferred from data collected in Jerlov (1976), Wame (1978) and Doak et al. (1980). In a Landsat image the spectral values corresponding to depths of penetration may be determined from the statistical distributions of the bands over clear deep water where no discernable signal is returned from the bottom. The upper limits to the deep water statistical variation may be used to divide the water covered areas of an appropriately pre-processed (see Jupp et al., 1985) image into three classes, or Band Zones (Bina et al., 1978). (a) Band 4 Zone Bands 5, 6 and 7 within the deep water limit but band 4 above it. This zone may be approximately associated with water depths between about 15 and 5 m when the water is clear. (b) Band 5 Zone Bands 6 and 7 within the deep water limit but band 5 above the upper limit and band 4 arbitrary. This zone may be approximately associated with clear water depths between 5 m and 50 cm. (c) Band 6 Zone Band 7 within the deep water limit but band 6 above the deep water

31 limit and bands 4 and 5 arbitrary. This zone is probably less than 50 cm deep, but is a difficult zone to map bathymetrically on reefs where in shallow water the variation in the water surface, protrusion of coral structure and variations in substrate type may dominate the depth effect. The use of the band 7 limit in each case ensures that the band zones are submerged areas of the image as total absorption of band 7 characterizes water. The Band Zone criteria may be used to separate the deep water from reefal shoals and shallow reef areas for classification, and provide broad depth classifications for resolving many depth/substrate confusions. Assuming the depths to the cut-off points are correct, or on the basis of field data and tide corrections, depths within the Band Zones may be consistently interpolated using Eq. (1) as follows: Let: X i = log(Li - L w i )

then, to the extent that Eq. (1) models Li (Lyzenga, 1981): X i = log(Lbi - L w i ) - 2 K i z

(2)

In the Band 4 Zone, the lower limit for band 4 is known by definition. By locating the optimum decision threshold between the statistical distributions of band 4 in the Band 4 and 5 Zones it is possible to define an upper limit for band 4 in the Band 4 Zone. If the depths to the two Landsat based decision boundaries are known then Eq. (2) allows and X i , and therefore an Li, value to be computed for any intermediate depth by linear interpolation. Similar steps can provide interpolated " d e p t h " slices for band 5 in the Band 5 Zone and band 6 in the Band 6 Zones (see Fig. 3}. The successful use of this m e t h o d depends on variations in band 4 in the Band 4 Zone, band 5 in the Band 5 Zone and band 6 in the Band 6 Zone being largely depth related. Such interpolated images are being prepared by the ASO for the whole of the GBR. Where depth control is sparse or unavailable the interpolated Depth of Penetration images are being produced as first order approximations with suitable qualifications. A test has been run on a well surveyed set of reefs off Innisfail in the Cairns Section of the GBR Marine Park. The image used had been classified and labelled as part of the exercise reported in Jupp et al. (1985a). This image was, unfortunately, not ideal as it was recorded in mid-winter when the low sun angle, by reducing the relative size of Lwat in the recorded signal, reduced the effective depth of penetration. The point depth samples read off the soundings made by the ASO Field teams were integrated into a channel of the image. Since they are point data they are not directly comparable with the integrated Landsat data so that a statistical approach to the depth calibration was adopted.

32 Histograms of the depth samples stratified by Band Zone were used to define decision boundaries between the overlapping distributions. It was found that in each of the Band Zones 80% of the samples fell within depth limits of 11.5 m, 4.0 m and 0.4 m (Bands 4, 5 and 6 respectively). Except for the cut-off between the Band 4 Zone and the deep water this is quite similar to the nominal values of 15.0 m, 5.0 m and 0.5 m. The shallower value for the Band 4 Zone limit was a function of the difficulty of resolving the deep water threshold under the poor light conditions. The interpolated images provided useful results within this reduced depth range except that turbid patches were mapped as shoals. There seems no way to separate bathymetry and turbidity in a single image except possibly manually, using spatial patterns. A series of tests which utilize existing surveyed reefs and the registered depth data from newly surveyed reefs together with the base of processed imagery and multi-date imagery as it develops is being planned to fully assess the benefits of the calibrated bathymetric imagery. 6. TOPOGRAPHIC VARIATIONAND TEXTURE Topographic variation in the form of sea floor slope and aspect can be inferred using Eq. (1) and numerical differentiation of the Landsat data. Assuming constant water properties: VLi/(Li - Lwi) = VLbi/(Lbi - Lwi) - 2Ki Vz

(3)

where: V is the gradient vector (a/ax, a/ay). That is, the relative gradient of the radiance may be separated into two components. One is due to reef cover change and texture and the other to topographic slope and aspect. The topographic component usually dominates the variation. The significance of the two components for reef cover classification led to a texture channel being incorporated into computer based classification of the Cairns Section of the Marine Park (Jupp et al., 1985a). Texture, or mean variance between a pixel and its neighbours is an RMS average gradient or non-directional RMS slope magnitude. The use of this channel effected many separations of a geomorphological nature. The general dominance of the topographic component in the gradient has led to the development of the "Exposure" image. Exposure to weather (E) from the direction (p,q) is defined to be: E(p,q) = - ( p aL4/ax + q aL4/ay)

(4)

Here, p and q are the direction cosines of the weather vector and Landsat band 4 is used for its depth penetration. For constant reef cover type (which can be promoted by selective image filtering) it follows from Eq. (1) that: E ( p , q ) -- 2 K 4 (L4 - Lw4)(P a z / a x + q a z / a y )

(5)

33 which relates E directly to topographic slope and aspect and leads to images like Fig. 4. The exposure image is simply the directional c o m p o n e n t of texture which is the RMS average of E over the compass. Besides providing somewhat dramatic enhancements of reef topography and fine visual detection of deeply submerged features, these images have a biological significance. Physically and biologically there is a close relationship between reef cover type, the b a t h y m e t r y as it relates to the tidal range and exposure to weather in the form of the prevailing winds. For most of the year the southeasterly trade winds provide the dominant energy vector, promoting and controlling both the growth and erosion of reefs through provision of nutrients and transport of sediment. Biota are distributed over reefs to a c c o m m o d a t e their preferences for cover type, depth range and exposure. Exposure images may therefore be used in conjunction with other information to infer reef biological patterns. The standard GBR Exposure image product uses the southeast wind as its vector. Exposure images with the wind vector from the north may also be used to assess vulnerability of reef areas to storms or cyclones (hurricanes) from that direction. 7. SEPARATING DEPTH AND SUBSTRATE TYPE The remaining information which may be obtained from Landsat imagery is reef cover type. This, together with position in the tidal range and exposure, would (if accurate} form a significant base of data for reef mapping and survey planning. Apart from disturbing effects such as atmospheric haze (that is, Latm and Ta) and scattering from the water surface (Lsurf), the main variables modifying the strong depth gradient in Eq. (1) are water colour, turbidity and b o t t o m reflectance. The b o t t o m reflectance, to the extent that it is separable in the recorded data from the four Landsat bands, is the available measure of cover type. Using the terms defined for Eq. (1) with i = 4 and 5, the following useful ratio {cf. Lyzenga, 1981) may be defined: Rsub

= (L5

- Lws)/(L4

-

Lw4)7

(6)

where: 3' is the ratio of the attenuation coefficients K s / K 4 . Rsu b is independent of depth and depends only on the substrate type to the extent that eq. (1) does represent the above water radiance. That is, from Eq. (1) it follows that Rsu b = (Lbs -

Lws)/(Lb,

, -

Lw4)T

(7)

Rsub therefore depends only on the (wet) reflectance of the sea floor (or substrate), the deep water reflectance (Lw4, Lws) and the attenuation ratio ~/(Ks~K4).

34 The attenuation ratio ~ depends on the water colour and turbidity and is n o t well established in the GBR region. Based on existing overseas data, it might be expected to range from a b o u t 4.5 d o w n to 2.5 in the types of water encountered around reefs and be as low as 1.1 in near-shore waters (Jerlov, 1976). However, where co-registered depth and Landsat data exist in the Band 4 and 5 Zones the attenuation coefficients K4 and K5 may be estimated using Eq. (2) in the appropriate Zone. The estimate for 7 so obtained can then be used in Eq. (6) to map shallow (Band 5 and 6 Zone) substrates which are approximately "depth free". For the Innisfail image used for classification and depth testing 7 was 3.34 (K4 = 0.0935 and Ks = 0.3118). This corresponds to Jerlov's Water Type II which may be a b o u t average for the GBR lagoon water. The reflectances of the materials comprising reefs are also poorly known and considerable work is needed to measure and analyze the spectral properties of both the reef waters and the (exposed) reef materials. If the band 5 radiance is graphed against the band 4 radiance, the mean value over deep water will approximate: DW = (Lw4,Lw5)

as defined b y Eq. (1). As the depth increases, band 4 and band 5 both eventually increase from the deep-water point with band 4 responding to the sea floor first. If the composition of the sea floor were uniform then the trajectory of this plot would be a smooth function of depth, starting at DW and ending at the (wet) radiances of the material comprising the uniform floor (Lb4, Lgs). In practice the non-uniform cover spreads the data in a way which represents the increasing radiance contrast between different covers as the water depth decreases. On reefs the separation as measured b y Rsub is very large and the types of cover present correlate with depth. This causes simple approaches to depth mapping (such as fitting exponentials to band data) to fail in shallow areas near and within reefs. From Eqs (6) and (7), it follows that: (L5 - Lw s) = Rsub(L4 - Lw4) ~/

(8)

so that the signature of a pixel with given depth and substrate t y p e may be associated with an appropriate point of the depth trajectory (Eq. (8)) for the substrate type as indexed by Rsu b. The effects of this theoretical model may be illustrated using Fig. 6. Figure 6 was developed using exposed reef c o m p o n e n t signatures of coral, sea grass and pure carbonate sand. These were collected by members of the team during field work on Arlington and Green Island reefs in 1980 using an EXOTECH model 100 A radiometer. The model parameters (Lw and K) were c o m p u t e d as described in Kendall and Jupp (1981) and the water absorption and scattering parameters were taken from Smith and Baker (1981).

35 Equation (1) was used to simulate the effect of increasing water cover on the band 4 and band 5 signatures assuming the water is pure salt water with no suspended solids or dissolved material. In Fig. 6, the curves joining the deep water point (Lw4, Lws) to the exposed c o m p o n e n t signatures (Lb4, Lbs ) follow Eq. (8) and the positions of the Band Zone boundaries are shown on the curves. The interaction between c o m p o n e n t mixing and water cover is indicated by the simulated plot for a mixture of 50% sand and 50% coral. 30 Pure Carbonate

Sand

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For constant water conditions, '1, is a constant and the substrate types separate on the substrate ratio Rsub. Rsub is an index of the reflectance contrast between reef covers relative to the deep water signal. However, the existence of water colour and turbidity variations, the effects of the atmosphere on the signal and the mixing of the reef components within a pixel make this separation much more difficult in practice. In addition, Rsub is only defined in the Band 5 and 6 Zones which contain the top 5 m of the reef. The use of this model in reef cover mapping had therefore to be verified in the field before being combined with the Depth of Penetration analysis to produce classification images. 8. M O D E L V E R I F I C A T I O N

The first-order correctness of the model was verified by field studies on Heron Island Reef and other selected reefs of the GBR. The Heron

36

Island Reef studies were the most extensive and are reported in Jupp et al. (1985a, b), Mayo et al. (1985) and in greater detail in Kuchler (1984b). Heron Island and its associated reef is one of 14 reefs of the Capricornia Section which lies off the Queensland coast near Gladstone. Together with the 6 reefs of the Bunker Group, these reefs form the southern GBR province and comprised the first Section of the GBR Marine Park to be declared. 175.5

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/

/ /

37 A s u m m e r o v e r p a s s at 0 9 0 3 (local t i m e ) o n N o v e m b e r 27 1 9 8 0 was clear to t h e e x t e n t t h a t 15 o f t h e 20 reefs o f t h e C a p r i c o r n i a S e c t i o n w e r e i m a g e d . T h e tide was o n e m e t r e a b o v e G l a d s t o n e d a t u m , w h i c h is at t h e M e a n L o w Water Neap point providing excellent conditions for reef mapping. T h e L a n d s a t d a t a w e r e classified i n t o s p e c t r a l classes using t h e B R I A N s y s t e m { J u p p et al., 1 9 8 1 b , 1 9 8 5 b ) . This s e g m e n t i n g used n o p r i o r a s s u m p t i o n s o n pixel similarity o t h e r t h a n spectral similarity. T h e classes w e r e s a m p l e d t o p r o v i d e 341 s a m p l e s f o r c o m p a r i s o n a n d s t u d y . T h e s a m p l e s have b e e n m a p p e d i n t o i m a g e r y , p h o t o g r a p h s a n d o n t o t h e r e e f surface as field sites a n d a c o m p l e t e c o m p a r i s o n b e t w e e n t h e results is c o n t a i n e d in Kuchler (1984b). When t h e s p e c t r a l classes w e r e labelled i n t o a r e e f c o v e r m o d e l o n the basis o f t h e field w o r k , a set o f g e o m o r p h o l o g i c a l l y b a s e d z o n e s e m e r g e d . T h e s e are d i s p l a y e d o n a b a n d 4 a n d 5 t w o - w a y p l o t in Fig. 7. T h e labels used in Fig. 7 are d e f i n e d in T a b l e 1. TABLE 1 Classes used in reef cover model for Heron Island Reef General Open water

Reef front Reef rim

Code

Label description

O

Ocean

DW

Deep water

RS SR

Reefal shoals Reef slope

FR

Reef front

RR (E) RR •

Reef rim (Exposed) reef rim

Outer reef fiat

ORF

Outer reef fiat

Lagoons

MDL SDL

Medium depth lagoon Shallow depth lagoon

Inner reef flat

Exposed sand

SF SZ

Sand flat Sand zone

SRT

Sandy reef top

ES RZ

Exposed sand Rubble zone

T h e n o m e n c l a t u r e a n d d e f i n i t i o n s o f t h e z o n e s are based o n K u c h l e r ( 1 9 8 4 a ) a n d h a v e b e e n called t h e M a j o r G e o m o r p h o l o g i c a l Z o n e s (MGZs) f o r reefs ( H o p l e y , 1982}. T h e y z o n e a r e e f spatially in t h e w a y s h o w n in

38 Fig. 8 together with a depth division which is generally related to the tidal range for the reef. There are significant differences in reef biology between the MGZs, both with respect to coral growth and the associated biota (Hiatt and Strasburg, 1960; Mather and Bennett, 1978). Some scientists have termed them macro-habitats for reef biota since the physical environments of the MGZs dominate the c o m m u n i t y factors at work in determining their biological composition. REEF Reef E

Reef Flat

Reef S l o p e

Cay

Reef

Flat

Wind and Wave Energy

Top Logoon

Reefl

lReef

• ,

High tide

I

I

Coral growth dominant

Organic frame

Corailine algae dominant

Clastic sediments

Fig. 8. Cross-section of a platform reef with the major geomorphological zones labelled. The geomorphological zonation contains both a compositional component (for example coral, sand and water depth) and a contextual component (for example reef slope, deep lagoon, outer reef flat and inner reef flat). It is not surprising, therefore, that this study found that Landsat spectral data alone could not separate classes on the contextual component. (The use of a texture channel improved this separation in the Cairns Section exercise reported in J u p p et al., 1985a). Figure 7 does, however, show a clear separation of the data into compositional classes based on depth trajectories and relative amounts of coral and sand -- as modelled in Fig. 6. This work has, we believe, brought together on a physical basis and in an easily specified form the important exploratory work by Smith et al. (1975a, b); Bina et al. {1978) and Bina and Ombec {1979). 9. C L A S S I F I C A T I O N

IMAGES

Classification images, which map the compositional zones of reefs, form natural complements to images showing depth and topographic structure. Together, these images could provide a classification into MGZs which is

39 b o t h compositional and contextual. As outlined above, there is sufficient structure in Landsat data to map generalized zones of varying composition in shallow areas. In J u p p et al. (1985a) c o m p u t e r based classification, with a texture channel, was used for reconnaissance mapping in the Cairns Section of the GBR Marine Park. The rationale was to provide reef scientists with enough c o m p u t e r based data to assemble the spectral classes into a model for the reef covers and extend their interpretations over large (and unknown) areas. The initial results were impressive {see Fig. 5), but were difficult to specify as standard products due to the requirement of skilled interpreters and the relative complexity of computer-based classification. This has led to the development of an interim product which is straightforward to specify, and may provide a basis for interpretation by reef scientists over extended areas of the GBR Marine Park. The Deep water and Band 4 Zones have been defined in the Depth of Penetration products. However, the Band 4 Zone may be further subdivided by interpolation for the Classification image. Combining this interpolated image with the exposure image, either as c o m p u t e r overlay or manual overlay, can provide a useful geomorphological and implicit biological classifications of the deeper areas. In the Band 5 and 6 Zones compositional zones may be mapped. In the Band 5 Zone, band 5 responds principally to depth variations and band 4 to substrate variations. Corals are dark in band 4 and sand is bright. Therefore, in the Band 5 Zone, depth may be interpolated consistently using band 5, leaving band 4 to separate substrate within those " d e p t h " classes. The procedure consists of subdividing the interpolated depth classes into t w o subzones based on high and low band 4 values corresponding to "sand" and "coral" respectively. A similar procedure completes the thematic image in the Band 6 Zone. Where field data are available and an estimate for 7 has been found, a similar depth/substrate separation could be made using Rsu b. However, the process is quite complex, requires field data and seems to give essentially the same results as the more easily specified process outlined above. The mean value of the set of pixels in each of the themes can be used to further seed a full compter-based classification and the products may be combined with a texture or exposure channel for further interpretation. In this way an initial interpretation of the general zones of reefs may be produced which provides both objectively specified current information and a basis for future interpretation as the information base and experience of reef scientists expands. 10. FUTURE REMOTE SENSING OF THE GBR While there has already been a considerable and valuable input to the information needs of the GBRMPA and reef scientists from satellite data,

40

there is still greater scope for remote sensing in the difficult and extensive GBR Region. The reefs are only fixed points in a multilevel dynamic system in which reef biota communicate between reefs through the movement of the water masses. While the survey of the reefs is basic to reef studies, significant gaps exist concerning the m o v e m e n t of water masses, currents, internal waves and fronts, shelf circulation, suspended solids, pollution dynamics, the biological productivity of the waters and the degree of inter-reef and reef-coast communication. In the future, integration of data from the whole reef system and from individual reefs as well as oceanographic data on reef communication will need data from active and passive instruments carried on a variety of platforms, including satellites, aircraft, ships and buoys. Particularly at the individual reef scale, data from airborne scanners, or more advanced instruments such as laser fluorosensors, have a significant role to play in reef studies and in providing data for planning and managing the GBR Marine Park. An initial survey of that potential can be found in Jupp (1984). Cartographically, there is considerable scope for using the higher resolution satellites such as Landsat 5 TM and the future SPOT satellite to improve the map base. The cartographic achievements of the Landsat TM TIPS system point to this as an immediate opportunity. 11. C O N C L U S I O N S

In summary, Landsat data have provided a mapping base for the GBR, interpretations of reef morphology, approximate b a t h y m e t r y adjacent to reefs and a set of plans for the whole GBR which express this potential. Additional uses for Landsat imagery of the GBR have included reef survey design, oceanographic studies (Wolanski et al., 1984) and suspended sediment studies along the Queensland coast. These have not been described in this paper, but underline the value of the Landsat series for reef mapping and for regional reef management. There is a growing awareness among reef managers and scientists, both in the GBR region and throughout the reef areas of the world, of the value of remote sensing to their studies. Only remote sensing, for example, may be able to cope fully with the related problems of assessing reef productivity and biological communications between reefs. The difficult nature of the GBR environment for survey and scientific study and the future needs of the GBRMPA also point to a considerable role for a wide range of remote sensing applications in the region. These range from oceanographic scale to individual reef scale and a wide range of future instruments and platforms will be needed to encompass this vast area.

41 12. ACKNOWLEDGEMENTS

Many people at the GBRMPA, the ASO and CSIRO as well as reef scientists from a number of locations helped to successfully conclude this work. In particular, Richard Kenchington of the GBRMPA perceived the value of remote sensing to his work and the Chairman of the GBRMPA, Graeme Kelleher, provided the encouragement and support needed to bring its many elements together. At CSIRO Division of Water and Land Resources special acknowledgement to Sandra Heggen, Stuart Kendall and Jenny Bolton for their excellent work and teamwork.

REFERENCES Bina, R. and Ombac, E., 1979. Effects of tidal fluctuations on the spectral patterns of Landsat coral reef imageries. Proc. 13th Int. Syrup. on Remote Sensing Environ., ERIM, Ann Arbor, Michigan, pp. 1293--1308. Bina, R., Carpenter, K., Zacher, W., Jara, R.S. and Lira, J.B., 1978. Coral reef mapping using Landsat data: follow up studies. Proc. 12th Int. Syrup. on Remote Sensing Environ., Ann Arbor, Michigan, Vol. 3, pp. 2051--2070. Claasen, D. Van R., Jupp, D.L.B., Bolton, J. and Zell, L.D., 1984. An initial investigation into the mapping of seagrass and water color with CZCS and Landsat in North Queensland, Australia. Proc. 10th Int. Symp. for Machine Processing of Remotely Sensed Data, Purdue University, West Lafayette. Doak, E., Livisay, J., Lyzenga, D., Ott, J. and Polcyn, F., 1980. Evaluation of water depth extraction techniques using Landsat and aircraft data. Final Report. Defense Mapping Agency, Hydrographic]Topographic Center, Washington DC, 1359000-2-f, 208 pp. Hiatt, R.W. and Strasburg, D.W., 1960. Ecological relationships of the fish fauna on coral reefs of the Marshall Islands. Ecol. Monogr., 30 (1): 65--127. Hopley, D., 1982. The Geomorphology of the Great Barrier Reef. Wiley-Interscience, New York, N.Y., 453 pp. Jerlov, N.G., 1976. Marine Optics. Elsevier, Amsterdam, 231 pp. Jupp, D.L.B., 1984. Report on the application and potential of Remote Sensing in the Great Barrier Reef region. GBRMPA Technical Report, GBRMPA, Townsville, Queensland. (+) Jupp, D.L.B. and Mayo, K.K., 1982. The physical basis for remote sensing of the Great Barrier Reef by Landsat. Workshop on Remote Sensing of the Great Barrier Reef, James Cook University, Townsville, Queensland. (+) Jupp, D.L.B., Mayo, K.K., Kuchler, D.A., Heggen, S.J. and Kendall, S.W., 1981a. Remote sensing by Landsat as support for management of the Great Barrier Reef. In: P. Laut (Editor), Proc. Second Australasian Remote Sensing Conference, Canberra, pp. 9.5.1--9.5.6. (+) Jupp, D.L.B., Mayo, K.K., Kuchler, D., Heggen, S.J. and Kendall, S.W., 1981b. The BRIAN method for large area inventory and monitoring. P. Laut (Editor), Proc. Second Australasian Remote Sensing Conference, Canberra, pp. 6.5.1--6.5.5. (+) Jupp, D.L.B., Guerin, P. and Lamond, W.D.D., 1982. Rectification of Landsat data to cartographic bases with application to the Great Barrier Reef region. Proc. URPIS 10, Australian Urban and Regional Information Systems Association, Sydney, Australia, pp. 131--147.

42 Jupp, D.L.B., Guerin, P., Mayo, K.K., Claasen, D. Van, R. and Kenchington, R., 1984a. Landsat as support for management of the Great Barrier Reef. In: E.W. Walker (Editor), Proceedings of the Third Australasian Conference on Remote Sensing, Queensland, pp. 706--716. Jupp, D.L.B., Guerin, P., Mayo, K.K., Claasen, D. Van, R. and Kenchington, R., 1984b. Remote Sensing based survey of the Great Barrier Reef of Australia. Working Group 3, Commission IV, International Society for Photogrammetry and Remote Sensing, RIO-84, Rio de Janeiro, Brazil. Jupp, D.L.B., Mayo, K.K., Kuchler, D.A., Heggen, S.J., Kendall, S.W., Haywood, M.J., Ayling, T. and Radke, B,M., 1985a. A Landsat based interpretation of the Cairns Section of the Great Barrier Reef Marine Park. Interpretation of Landsat data by computer based classification and labelling. CSIRO Division of Water and Land Resources, Natural Resources Series, No. 4, CSIRO, Melbourne. Jupp, D.L.B., Mayo, K.K., Heggen, S.J., Kendall, S.W., Bolton, J. and Harrison, B.A., 1985b. The BRIAN Handbook. An intorduction to Landsat and the BRIAN (Barrier Reef Image ANalysis) System for users. CSIRO Division of Water and Land Resources, Natural Resources Series, No. 3, CSIRO, Melbourne. Kendall, S.W. and Jupp, D.L.B., 1981. LIGHT - - a computer program for estimating radiative transfer through water and properties relevant to remote sensing of the sea floor. CSIRO Division of Land Use Research. Tech. Memo., TM 81/12. Kuchler, D.A., 1984a. Geomorphological nomenclature: Reef cover and zonation, Great Barrier Reef, Australia. Technical Report, Great Barrier Reef Marine Park Authority, Townsville, Queensland. Kuchler, D.A., 1984b. Geomorphological Separability, Landsat MSS and Aerial Photographic Data: Heron Island Reef, Great Barrier Reef, Australia. Ph.D. Thesis, James Cook University of North Queensland, Australia. Lyzenga, D.R., 1981. Remote sensing of b o t t o m reflectance and water attenuation parameters in shallow water using aircraft and Landsat data. Int. J. Remote Sensing, 2: 71--82. Mather, P. and Bennett, I., 1978. A coral reef handbook. Handbook Series No. 1, Great Barrier Reef Committee, Brisbane. Mayo, K.K., Jupp, D.L.B., Heggen, S.J. and Kendall, S.W., 1985. Heron Island Field data -- Field notes and data sheets. CSIRO Division of Water and Land Resources Technical Memorandum, 85/6. i Smith, E.V., Rogers, R.H. and Reed, L.E., 1975a. A u t o m a t e d mapping and inventory of Great Barrier Reef zonation with Landsat data. Ocean '75 Conference and Exposition. San Diego, California. Smith, E.V., Rogers, R.H. and Reed, L.E., 1975b. Thematic mapping of coral reefs using Landsat data. Proc. 10th Int. Symp. Remote Sensing Environ., Vol. 1, pp. 585--594. Smith, R.C. and Baker, K.S., 1981. Optical properties of the clearest natural waters (200--800 nm). Appl. Optics, 20 (2): 177--184. Warne, D.K., 1978. Landsat Image Analysis: Application to Hydrographic Mapping. Ph.D. Thesis, The Australian National University, Canberra. Wolanski, E., Pickard, G.L. and Jupp, D.L.B., 1984. Topographic waves, river plumes, eddies and mixing on the Northern Great Barrier Reef continental shelf in summer. Estuarine Coastal Shelf Sci., 18 : 291--314. (Reports marked (+) are available from the Publications Section, CSIRO Division of Water and Land Resources, Canberra, Australia as two Technical Memoranda TM 81133 and TM 84]8.)