Wednesday, December 14, 2016

Hydrology - Flow Direction 2/3

This is the second installment of the Hydrology trilogy focusing on the Flow Direction tool. See part one here and part three here. As the title suggests, this tool calculates the direction of water flow on a surface. As long as a cell is not located along the edge of a dataset, it has eight neighboring cells, and therefore eight different directions to flow, and will move to the neighboring cell with the greatest negative elevation difference over distance. The only exception is if there are no neighboring cells with a lower elevation, which means it may be a sink, and will be unclassified. Distance is measured from the center of each cell and, diagonal cells are therefore at a greater distance from the center cell. This means that the flow direction may not be calculated purely on the greatest difference in elevation.

The cells that have the same flow direction and are located next to each other create clusters of cells with the same color. These cluster creates patterns, and it is possible to identify the mountain area in the lower left corner.