CAPABILITIES OF LANDSAT-5 THEMATIC MAPPER (TM) DATA IN REGIONAL MAPPING AND CHARACTERIZATION OF INLAND VALLEY AGROECOSYSTEMS IN WEST-AFRICA

This study demonstrated the capabilities of Landsat-5 Thematic Mapper (TM) data in regional mapping and characterization of inland valley (v alley bottoms plus valley slopes or fringes) agroecosystems. A methodo logy was developed involving image enhancement, display, and digitizin g, as well as image and Geographical Information System (GIS) manipula tion techniques for rapid studies of inland valley characteristics ove r large areas. A case study was conducted using Landsat-5 TM data over the regions of Save and Parakou in the Republic of Benin (path:192, r ow:54 of Landsat TM World Reference System) encompassing about 3.12 x 10(6) ha (study area) to highlight the strengths of the methodology. T he results showed the ability of TM data, using the espoused methodolo gy, to map inland valley characteristics: (rr) as well as or better th an 1 :200000 topographic maps; and (b) to a significantly similar leve l to that of 1 :50000 topographic maps. The technique involving a rati o image of TM4/TM7 (red), TM4/TM3 (green), and TM4/TM2 (blue) when dis played in a magnifying factor of one or greater provided the single be st enhancement and display technique for mapping inland valley bottoms (see illustrations in figures 4, 5, and 11). A combination of this te chnique with two other image enhancement techniques, involving a false colour composite (FCC) of TM4 (red), TM3 (green), and TM5 (blue), and a ratio RGB (red-green-blue) image of TM4/TM5, TM4/TM3 and TM4/TM1, i mproved the results significantly. Relative to 1 : 200000 topographic maps TM data mapped up to 118 per cent stream frequencies (number of s treams km(-2)) and 99 per cent stream densities (length of streams km( -2)). However, relative to 1 : 50000 topographic maps TM data mapped o nly 74 per cent stream frequencies and 77 per cent stream densities. T he ability of TM data to provide the shape, size, area, spatial distri bution, and land use characteristics of inland valleys (see figure 11, for example), unlike other data sources, such as topographic maps, ha ve been highlighted in this paper. TM-derived valley bottom widths and vegetation characteristics (e.g., Ratio Vegetation Index; RVI) of val ley bottoms and valley fringes provided highly positive correlations o r formed statistically significant relationships with the correspondin g ground-measured information. In the total study area of 3.12 x 10(6) ha, covered by a full Landsat-5 TM scene (image), there were 9 per ce nt valley bottoms (see figure 11), and 19.2 per cent valley fringes as mapped using the espoused methodology. Of this, 7.9 per cent of the v alley bottoms, and 15.9 per cent of the valley fringes were cultivated . The percentage of inland valleys and their cultivation intensities r elative to their distance from the road network and settlements were e stablished. The major road network and a significant proportion of the settlements were also mapped directly from the TM data. These results indicate the strength of the methodology to rapidly map and character ize inland valley agroecosystems at regional level using high-resoluti on remotely sensed data.