The primary data source for hydrologic operations in ArcGIS is a raster of flow direction. This raster is formed by the FlowDirection calculation based on a surface of elevation; for our discussion here, we will call the resulting raster “DirOfFlow.” Each cell in the DirOfFlow raster contains an integer number; these numbers are powers of 2: 1, 2, 4, 8, 16, 32, 64, and 128. (Just why these numbers were chosen, rather than 1, 2, 3, etc., has a historical and computer component, which will be discussed.) Each number indicates a direction, as shown by Figure 8-21.
The idea is, simply, that the precipitation that falls, or otherwise appears, on a given cell flows immediately to a single adjacent cell. To which of the eight adjacent cells? The one indicated by the number and the arrow in Figure 8-21, which points in the direction of the steepest descending slope.
For example, consider the simple raster shown in Figure 8-22. The numbers in the cells indicate elevation.
The range in altitude is from 100 to 91, sloping gradually from east to west and a bit from north to south. When the Flow Direction calculation is applied to this raster, the resulting raster looks ...