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

The eight pixels in the “3 x 3 search-window” are all examined to see:o if their value is larger than zero - the algorithm should only examine cellscontaining riverso if the value is lower or equal to the current pixel’s value - the search should notbe able to proceed up-streamo if the pixel has been examined earlier in the ten-pixel loop - in order not to getstuck in a loop between two cells with the same pixel valueThe pixels, which fulfill these criteria, are then examined to calculate the best heightdifference from the current pixel, in order to know which pixel to go next. The calculatedheight difference is saved into a variable, which accumulates for each new cell. As can beseen in Figure 4.9, Iteration 1.2, the new pixel where to start from is at row three andcolumn three. The whole process is repeated until ten connected cells have been visitedor until the “river” ends, which is illustrated in Figure 4.9, Iteration 1.9. If theaccumulated height value is 20 meters or more, the visited cells, which coordinates havebeen stored in a temporary variable, are stored into the matrix “OUTPUT”, which canbe seen in Iteration 1 in Figure 4.9. In the case of Iteration 2 in Figure 4.9, the river-cellsaccumulated height value does not reach the required value of 20 meters and aretherefore not stored in “OUTPUT”. Iteration14 in Figure 4.9 illustrates the course ofevents when the search begins in the last pixel in a river. In this case there is only oneconnecting river cell in the “3 x 3 search window”, but since it is upstream, the searchwill be aborted and no output is stored.Inord er for the algorithm to present a correct result, anassum ption that rivers or streams never branch offdownstream in hilly areas has been made. The reason forthis assumption is that our algorithm in such cases (seeFigure 4.10) is limited since it only chooses the branch withthe best height difference and leaves one branchunattended. If this branch individually has a heightdifference of 20 meter it will also be stored in “OUTPUT”.In cases of same height difference the algorithm choosesthe last found branch.Figure 4.10 The unlikelyphenomenon of downstreambranching off rivers43