Africa Topographic Position

The Topographic Position dataset identifies two classes: uplands and lowlands/depressions. This is a derivative product of the Compound Topographic Index (CTI) which is a topographically derived measure of slope for a raster cell and the contributing area from "upstream" raster cells, and thus expresses potential for water flow to a point. Uplands and lowlands/depressions areas were mapped by determining a range of CTI values for uplands and for lowlands/depressions and then recoding the CTI dataset using two codes representing these ranges. The calculation of the CTI required two input data, flow accumulation and slope data. Flow accumulation was calculated from the 3 arc-second Drainage Direction dataset available from the HydroSHEDS database (Lehner et al., 2008; World Wildlife Fund, 2008). Slopes were calculated from the 90m SRTM elevation data created by void-filling and re-sampling the 30m SRTM elevation data provided by the National Geospatial-Intelligence Agency. Using these input datasets, the CTI calculation was done in a Python script (Worstell, 2008). The threshold value which defined CTI ranges for uplands and lowlands/depressions was decided using an additional dataset, the SRTM River-Surface Water Bodies dataset (Jenness et al., 2006), which mapped inland water body boundaries. Lake water body boundaries were overlain with the CTI data. CTI values which occurred over the boundaries were extracted, and the frequency distribution of these extracted values was calculated. The mean value in this distribution was used as the threshold. The CTI dataset was recoded into to two classes to produce the Topographic Position dataset.