Updated December 1998
by J. Populus, D. Lantieri
43 pp, 8 figures, 4 tables
RSC Series No. 58, FAO, Rome 1991.
After the possibilities of high-resolution satellite data, mainly SPOT, for tropical coastal fisheries were investigated, it was concluded that these data could be used to provide updated 1:50,000 maps and related statistics of a number of features of prime interest for coastal fisheries management, covering particularly:
Coastline. It was shown that a single visual interpretation of the satellite imagery led to an excellent and geometrically correct baseline map of the coast;
Intertidal space. In the pilot study, both active and idle aquaculture ponds could be shown as well as main vegetation categories such as grasslands and healthy and degraded mangroves. The possibility of mapping these features was confirmed by results of other studies in Ecuador, Brazil, Bangladesh and Kenya.
Shallow waters (up to 10 m deep) could be characterized by means of the following:
Bottom types, that is to say, whether covered by sand, grass or coral. Discrimination can be made in certain cases between sparse and dense grass and between live and damaged coral although it could not be demonstrated in this pilot study for lack of field measurements (e.g. different coral types in French Polynesia or extension of seagrass in Europe);
Bathymetry. Several studies showed that in areas where bottom types were well discriminated and mapped, several bathymetric lines could be drawn with reasonable confidence, (0.5m) for up to 20 to 40 m depending on the turbidity of the water and the satellite (Landsat TM band I is supposed to be more penetrant), The intervals between lines vary from 2 to 5 m. In this pilot study, two lines showing areas between 3 to 3.5 m and 10 to 13,9 m respectively were mapped on a scale of 1:50,000.
From the technical point of view, satellite data analysis in coastal zones is based on both digital and visual analysis. Thus digital processing is an absolute necessity in order to analyse features of the inland, intertidal and shallow-water zones. This processing is done in several steps:
Data correction. Before being analysed, digital data must be corrected radiometrically and geometrically. Radiometric distortions, due mainly to atmosphere, can be removed using special model-based software, already available from several specialized institutions. Geometric distortions are well known (earth curvature, satellite motion) and can be easily corrected digitally in order to produce satellite imagery superposable on any topographic map.
Data interpretation. Due to the high dynamic range of reflectance occurring in a coastal zone, it is necessary to analyse separately the inland, the intertidal and the shallow-water zones, in order to allow a full display of the image content. The inland area is identified by computer-assisted visual analysis, a task which appeared to be the most time-consuming. The intertidal area is differentiated from shallow water using simple digital masking techniques on the channel near infra-red (XS3 for SPOT, TM4 for Landsat TM). The shallow water spreads between the intertidal line/waterline and the offshore limit of XS1 (or TM2) visibility. Land features of the inland and intertidal zones can be identified and mapped either through visual interpretation of the image concerned (inland or intertidal) after digital enhancement or through supervised classification. Good results were obtained in the pilot study from thresholding a vegetation index.
In the marine areas, bottom types and bathymetry lines could be mapped using a bottom type index Y = K2 log (XS1 - XS1oo) - K1 log (X2 - X2oo) where K1 and K2 are the water attenuation coefficient in bands XS1 and XS2 determined in the field by radiometric measurement, and Xsoo and XS2oo the maximum depth of light penetration for XS1 and XS2 respectively. These measurements have to be made in each bottom type and need well-localized training-set areas in the sea. This task was found difficult and requires well-trained personnel, a suitable boat and, if possible, oblique aerial photographs.
Finally, maps and statistics devised from high-resolution satellite data were found very useful for applications such as the following:
From the economic point of view, the production of maps in the coastal area is 25 to 35 percent cheaper with high-resolution satellite data than with black and white or colour aerial photos. For monitoring purposes, satellite data are even cheaper. They also seem more advantageous when their superiority in mapping bathymetry and bottom types is considered. The time needed to draw a map, however, seems to be much the same using satellite data or aerial photos.
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