International Journal of Remote Sensing Cover ...

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International Journal of Remote Sensing

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Cover Mapping and measurement of tropical coastal environments with hyperspectral and high spatial resolution data C. D. CLARK; H. T. RIPLEY; E. P. GREEN; A. J. EDWARDS; P. J. MUMBY

To cite this Article CLARK, C. D. , RIPLEY, H. T. , GREEN, E. P. , EDWARDS, A. J. and MUMBY, P. J.(1997) 'Cover

Mapping and measurement of tropical coastal environments with hyperspectral and high spatial resolution data', International Journal of Remote Sensing, 18: 2, 237 — 242 To link to this Article: DOI: 10.1080/014311697219033 URL: http://dx.doi.org/10.1080/014311697219033

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int. j. remote sensing, 1997, vol. 18 , no. 2 , 237± 242

Cover M apping and measurement of tropical coastal environments with hyperspectral and high spatial resolution data C. D. CLARK² , H. T. RIPLEY §, E. P. GREEN³ , A. J. EDWARDS³ and P. J. MUMBY²

Downloaded By: [University of Queensland] At: 11:40 13 November 2010

² She eld Centre for Earth Observation Science & Department of Geography, University of She eld, She eld, S10 2TN, England, U.K. ³ Centre for Tropical Coastal Management Studies, Department of Marine Sciences & Coastal Management, University of Newcastle, Newcastle upon Tyne, NE1 7RU, England, U.K.

§Ariel Geomatics Incorporated, One Research Drive, Dartmouth, Nova Scotia, B2Y 4M9, Canada

1.

Description

The cover image (® gure 1 ) displays data acquired by mounting a CASI (Compact Airborne Spectrographic Imager) sensor onboard a light aircraft (see table 1). Recent enhancements to the hardware and software of the CASI system have produced a signi® cant 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. Utilizing this new potential, south to north ¯ ights 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. The east and west edges of the image are curvilinear because each scan line has been relocated to adjust for the e ects of the aircraft. This was achieved using data from an onboard gyroscope which monitored the lateral roll during ¯ ight. 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 (annotated by yellow crosses in ® gure 2). These are high biomass mixed stands of the turtle grass T halassia testudimum, and manatee grass Syringodium ® liforme ( ® gure 3). The high spatial resolution permits invidivual `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 0143± 1161/ 97 $12.00

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1997 Taylor & Francis Ltd

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( Zieman 1982 ). The red crosses (® gure 2) mark two large blow-out features. Much of the lighter brown in the image represents lower biomass seagrass stands. The textured brown and tan patch with the blue cross at its centre marks a zone of large coral heads, dominated by the species Montastrea annularis and Porites porites (® gure 4). 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 colouration around the southern shores of Dove Cay demarcates a more diverse reef habitat with large colonies of Acropora palm ata dominating the

Figure 1. Airborne multi-spectral image of seagrass and reef habitats o the shore of South Caicos, British West Indies. 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 maximize colour separability.

Cover Table 1. Sensor Spatial resolution Spectral resolution


Platform Altitude Date

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Image acquisition details.

CASI (Compact Airborne Spectrographic Imager) 1m 8 wavebands; 2 blue, 2 green, 2 red, 2 NIR, varying between 8 and 20 nm in width. Waveband settings (nanometres): 402´5± 421´8; 453´4± 469´2, 531´1± 543´5, 571´9± 584´3, 630´7± 643´2, 666´5± 673´7, 736´6± 752´8, 776´3± 785´4 Cessna 172N (a single engine, high wing light aircraft with four seats) 2750 ft ( 840 m) 10 a.m. 16 July 1995

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 owned and not speci® cally adapted for aerial survey work, and so had no observation hatch. We mounted the sensor by ® tting a specially designed door with incorporated mounting brackets and a streamlined cowling. Power for the operation of the sensor was provided by portable batteries, as the electrical supply of the aircraft was inadequate. An incident light sensor was ® xed

Figure 2. Annotated version of ® gure 1, with crosses representing speci® c habitats as described in the text: yellow = seagrass meadows; red= `blow outs’ within the seagrass; blue = Montastrea reef habitat; black = soft corals on bare substrate.


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Figure 3. Edge of a seagrass `blow-out’ in a T halassia dominated mixed meadow of Thalassia and Syringodium (see red cross in ® gure 2). Note the exposed sand substrate, and wall (ca 70 cm high) of seagrass rhizomes and trapped sediment at the blow-out edge.

Figure 4.

Montastrea reef habitat. Colonies are 1± 2 m in diameter at their bases. Photograph was taken close to blue cross in ® gure 2.

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to the fuselage so that simultaneous measurements of irradiance could be made. Pro® les of irradiance and temperature change between ground level and the image acquisition altitude were also recorded. A DGPS ( Di erential Global Positioning System) was mounted to provide an accurate record of the aircraft’s ¯ ightpath. The use of a standard aircraft and pilot with the adaptations mentioned above permitted us to acquire image data at a fraction of the cost of more usual methods which utilize specialist survey aircraft, instrumentation and experienced survey pilots. The CASI data presented here is only a small part of that collected. 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, and 3 m pixels 3. Marine bandsetting: 16 wavebands optimized for water penetration, with 3 m pixels 4. T errestrial bandsetting: 16 wavebands optimized for mangrove mapping and assessment, with 3 m pixels 2.

Context

The coastal environment and its resources are under strong pressure from population growth, industrialization and tourism throughout the developing world. Integrated management is widely recognized as the basis for sustainable use. The ® rst step towards this often involves an evaluation and mapping of the resources using remote sensing techniques ranging from aerial photography to satellite imagery. Such an approach can be an e ective means for a developing nation to improve its management of coastal resources. Despite the increasing use of satellite imagery, there has been no rigorous analysis of the relative costs and bene® ts of using di erent imagery types, or an outline of what objectives can practically be achieved using a given technology with a given level of inputs (e.g., ground truthing, image processing). For a recent review of the use of remote sensing for tropical coastal resource assessment see Green et al. ( 1996 ). Without such analysis it is not possible for planners or decision makers in developing nations to decide: 1. When the use of remote sensing is likely to be appropriate and cost-e ective, and, if it is to be used. 2. Which type of imagery would be appropriate for di erent objectives. 3. The probability of data being already available in archive or acquirable within a project time frame. 4. The costs involved not only in acquisition but in processing and interpretation to a desired level of accuracy. 5. Potential economic bene® ts. The objectives of our research are to evaluate satellite and airborne remote sensing with respect to their abilities to achieve a suite of de® ned coastal resource management objectives. This involves technical evaluation in terms of how accurately resources can be mapped and measured, and an economic evaluation that documents the costs of achieving these outputs. 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

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over a large part of the Caicos Bank, situated in the Turks and Caicos Islands of the British West Indies ( 71ß W, 21ß N ). Imagery being evaluated include Landsat MSS and TM, SPOT XS and Panchromatic, ERS-1 SAR, aerial photography, and airborne CASI. CASI data were obtained to provide a high resolution best-case assessment of 2 `truth’ for relatively small ( 10 km ) areas subject to intensive ® eldwork (>200 persondays). Together, these provide detailed quantitative information on habitat extent and composition, water depth, seagrass biomass, mangrove canopy 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. Acknowledgments

This research is funded by the U.K. Overseas Development Administration’s Environment Research Programme. We are very grateful to the Turks and Caicos Islands’ Ministry of Natural Resources for their logistical assistance in our ® eldwork, and in particular to Mr John Ewing, Mr Christie Hall, Dr Paul Medley, Mr Chris Ninnes, and Mr Perry Seymore, and to Jean-Pierre Angers for the loan of the modi® ed Cessna door. Angie Ellis is acknowledged for her untiring assistance in the ® eld, and Dr Steve Plummer for some useful advice. References G reen, E . P ., M umby, P . J ., E dwards, A . J ., and C lark, C . D ., 1996. A review of the

application of remote sensing for tropical coastal resource assessment and monitoring. Coastal Management, 24, 1± 40. Z ieman, J . C ., 1982, The ecology of the seagrass of South Florida: a community pro® le. U.S. Department of the Interior, Bureau of Land Management, Fish and Wildlife Service ( FWB/OBS-82/55).