The direction of flow is determined by the direction of steepest descent from each cell. This is calculated as:
change in z-value / distance * 100
The distance is calculated between cell centers. Therefore, if the cell size is 1, the distance between two orthogonal cells is 1, and the distance between two diagonal cells is 1.414.
If the descent to all adjacent cells is the same, the neighborhood is enlarged until a steepest descent is found.
If all neighbors are higher than the processing cell, it will be considered noise, filled to the lowest value of its neighbors, and have a flow direction toward this cell. However, if a one-cell sink is next to the physical edge of the raster or has at least one NoData cell as a neighbor, then it is not filled due to insufficient neighbor information. To be considered a true one-cell sink, all neighbor information must be present.
If two cells flow to each other, they are sinks and have an undefined flow direction. This method of deriving flow direction from a digital elevation model (DEM) is presented in Jenson and Domingue (1988).
Greenlee, D. D. 1987. Raster and Vector Processing for Scanned Linework. Photogrammetric Engineering and Remote Sensing
53 (10): 1383–1387.
Jenson S. K. and J. O. Domingue. 1988. Extracting Topographic Structure from Digital Elevation Data for Geographic Information System Analysis. Photogrammetric Engineering and Remote Sensing
54 (11): 1593-1600.
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