Accuracy validation can be achieved in two ways: spatial overlays and specific area comparisons. The 1 KM DEM derived basins were developed by comparing with a control source developed by the Food and Agriculture Organization (FAO, 1995) which was overlayed upon the DEM sinks and basins during the verification and development stages. The control source was used as a spatial template to give us an independent indication of where the actual basins would be located on the ground. The FAO source was hand-digitized/verified from Defense Mapping Agency topographic maps at a scale of 1:2,000,000, which is comparable to the 1:1,000,000 scale 1 KM DEM. The Figure below for the Congo-Zaire Basin reports the 1 KM basin area compared to nine other sources. Extensive review has identified the previous nine sources as having inconsistent methodologies. As an example the graph displays the source "Czaya", who used 1:5,000,000 scale source maps to delineate the drainage basins for Africa (Sharpe, 1994). This scale compared to the 1 KM is far too coarse for detailed analysis of basins at the continental level. Overall, the 1 KM source and FAO compared fairly well with only a discrepancy of 0.1%, while the rest of the sources produced varied results depending on each source.
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