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# Calculating the Length of a Potential Linear Water Body

The length of a potential creek or stream is a useful thing to know when modeling. You can apply the Flow Length calculation to the direction of flow raster to show either the length of the channel of flowing water from each cell upstream or downstream. Upstream flow length for a given cell is the distance, totaled from cell to cell, from the given cell to the origin of the longest path of water (the top of its basin) coming into that cell. Downstream flow length from a given cell is the distance from that cell to the pour point for the water passing through the given cell. See Figure 8-30, where darker shades indicate larger flow length.

The upstream flow length looks like Figure 8-31, where darker shades indicate longer flow lengths.

The Input Weight Raster in the Flow Length tool operates in precisely the same way as the weight raster does (impedance, cost surface) in our previous discussion of proximity. It multiplies the length through each given cell by the value in the geographically equivalent cell in the weight raster. The weight raster provides the cost or impedance for water to flow through each cell. Thus, you could simulate the fact that water ...

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