Finding Potential Sites for Small-Scale Hydro Power in Uganda: a ...

The primary erosive force determining the general shape of most landscapes is water. Forthis reason, most landscapes have many hilltops and few sinks leading to a connecteddrainage pattern. This knowledge about surfaces is used in the ANUDEM technique inTOPOGRID, a command used in ArcGIS. It is used by imposing constraints on theinterpolation process resulting in connected drainage structure and correct representationof ridges and streams (ARC/INFO).The method consists of an iteration technique, which employs a nested grid strategy. Itstarts from an initial coarse grid and successively calculates grids at finer resolutions. Ithalves the cell size until the final user defined resolution is obtained. The initial values forthe first coarse grid are calculated by using the heights of local maxima based on thesurrounding contour height information, since no other information on the maximumheight is available. The following finer grid resolutions are based on linear interpolationfrom the preceding coarser grid (Asserup & Eklöf, 2000).The ANUDEM technique enables a drainage enforcement algorithm, which is designedto remove all sink points in the DEM that has not been identified as sinks in the inputdata. The sinks are being found by comparing the height of each grid point with theheight of the eight immediate neighbors (ARC/INFO; Wahba, 1990).The procedure of this interpolation has been designed to take advantage of the types ofinput data commonly available, and the known characteristics of elevation surfaces. It isoptimised to have the computational efficiency of ‘local’ interpolation methods such asInverse Distance Weighted interpolation, without losing the surface continuity of globalinterpolation methods such as Kriging and Splines (Hutchinson, 1989). Streamlines aswell as contours are widely available from topographic maps and provide importantstructural information about the landscape. ANUDEM can use such streamline data,provided that the streamlines are digitised in the downhill direction (Wilson and Gallant,2000).The result from the example interpolation done with ANUDEM with the input datashown in Figure 4.3 can be seen in Figure 4.4. ANUDEM generates ridges andstreamlines from points of locally maximum curvature on contour lines. This permitsinterpolation of the fine structure in contours across the area between them. This is donein a more reliable way than methods such as Kriging and Spline (Hutchinson 1988). Thederived contours also show a close match with the contours in the input data in Figure4.3 (Gallant & Wilson 2000).36