Mapping and measurement of tropical coastal environments with hyperspectral and high spatial resolution data

Chris Clark Herb Ripley Ed Green Al Edwards Pete Mumby

Description

The image to the right is an airborne multi-spectral image of seagrass and reef habitats of the shore of South Caicos, British West Indies acquired by mounting a CASI (Compact Airborne Spectrographic Imager) sensor onboard a light aircraft (see Table 1). The small island in the south is Dove Cay. The very high resolution (1 m by 1 m pixels), permits 'blow-outs' in the seagrass meadows and individual coral reef heads to be detected. The image is a colour composite using two blue and one green waveband and has been decorrelation stretched to maximise colour separability. Specific habitats are marked on the clickable map:

yellow crosses = seagrass meadows;

yellow circle = seagrass blow-out;

cyan crosses = calcareous green algal habitat;

magenta crosses = soft corals on bare substrate;

red circle = Montastraea reef habitat;

light green star on headland = view southwards across area of image.

Recent enhancements to the hardware and software of the CASI system have produced a significant reduction in the integration time, which has improved its overall spectral and spatial capabilities. it is now possible to acquire square pixels of 1 m resolution, and to acquire a higher number of wavebands for a given swath width.

Utilising this new potential, south to north flights were made over Cockburn Harbour, South Caicos Island, British West Indies. Individual roof-tops, trees, and large coral heads are visible and a boat can be observed moored alongside a jetty. Eight wavebands were chosen between 402 and 785 nm using wavelengths that would provide good water penetration and also be of use for infrared sensing of mangrove and other terrestrial vegetation (Table 1). The east and west edges of the image are curvilinear because each scan line has been relocated to adjust for the effects of the aircraft. This was achieved using data from an onboard gyroscope which monitored the lateral roll during flight.

As with many tropical waters that are remote from sources of suspended sediment such as rivers, the clarity of the water is exceptionally good. Water depths vary from a few centimetres at the coastline to over 15 m in the southwest of the image, although for most of the area the depth is less than 5 m.

Whilst some of the variation in colour results from bathymetric variation the dominant control is the nature of the marine habitats. Large meadows of seagrass are evident (marked by yellow crosses) in oblique aerial photographs and the CASI imagery.

These are high biomass mixed stands of the turtle grass Thalassia testudinum, and manatee grass Syringodium filiforme. The high spatial resolution permits individual 'blow-outs' (Zieman 1982) to be detected, which may vary between a few metres in diameter to tens of metres. They are important for seagrass dynamics as they provide a record of destructive events, and may act as a foci for seagrass loss and erosion of the substrate (Zieman 1982). The yellow circle marks a large blow-out feature. Much of the lighter brown in the image represents lower biomass seagrass stands.

The textured brown and tan patch with the red circle at its centre marks a zone of large coral heads, dominated by the species Montastraea annularis and Porites porites . The high resolution permits the mapping of this reef habitat and even the location of individual reef heads which are typically only 1-3 m in diameter. The brown-red coloration around the southern shores of Dove Cay demarcates a more diverse reef habitat with large colonies of Acropora palmata dominating the near shore zone. The darker blue in the south-west of the image represents the deeper waters of so-called Shark Alley.

The aircraft was locally loaned and not specifically adapted for aerial survey work, and so had no observation hatch. We mounted the sensor by fitting a specially designed door with incorporated mounting brackets and a streamlined cowling. Power for the operation of the sensor was provided by a heavy-duty aircraft battery, as the electrical supply of the aircraft was inadequate. An incident light sensor (ILS) was fixed to the fuselage so that simultaneous measurements of irradiance could be made. Profiles of irradiance and temperature change between ground level and the image acquisition altitude were also recorded. A DGPS (Differential Global Positioning System) was mounted to provide an accurate record of the aircraft's flightpath. The use of a local aircraft and pilot with the adapted door mentioned above allowed us to acquire image data at a fraction of the cost of more usual methods which utilise specialist survey aircraft, instrumentation and experienced survey pilots.

The CASI data presented here is only a small part of that collected by the project: The following missions were executed:

1. High spatial resolution: 1 m pixels, 8 wavebands (as illustrated)
2. Hyperspectral: 288 wavebands for a 120 m swath, with 3 m pixels; and 144 wavebands for a 300 m swath, with 3 m pixels.
3. Marine bandsetting: 16 wavebands optimised for mangrove mapping and assessment, with 3 m pixels.

Context

The work is restricted to tropical coastal environments and is mostly concerned with reef and seagrass habitats, coastal wetlands and mangroves. Our test site extends over a large part of the Caicos Bank (see image to the left), situated in the Turks and Caicos Islands of the British West Indies (71° W, 21° N). Imagery being evaluated include Landsat-MSS and TM, SPOT XS and Panchromatic, aerial photography, and airborne CASI.

CASI data were obtained to provide a high resolution best-case assessment of 'truth' for relatively small (10 km²) areas subject to intensive fieldwork (>200 person-days). Together, these provide detailed quantitative information on cover etc. This is being used to test the accuracy of outputs derived from analysis of coarser spatial and spectral resolution satellite imagery. In addition the ability of CASI to provide detailed information relating to habitats and communities of interest to coastal managers is being assessed.

References

	GREEN, E.P., MUMBY, P.J., EDWARDS, A.J., and CLARK, C.D. 1995, A review of the application of remote sensing for tropical coastal resource assessment and monitoring. Coastal Management, 24, 1-40.
	ZIEMAN, J.C., 1982, The ecology of the seagrasses of South Florida: a community profile, U.S. Department of the Interior, Bureau of Land Management, Fish and Wildlife Service (FWB/OBS-82/55)

Acknowledgements

© A.J.Edwards, 1997