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You will now undertake a series of geoprocessing steps to understand where water will flow and how much will accumulate. Essentially, you will be learning how to create flowlines and watershed boundaries, similar to those you downloaded from NHDPlus, based on DEM data.
Filling topography sinks
The Fill tool fills sinks in the DEM raster to remove small imperfections in the data. If cells with higher elevations surround a cell, the water gets trapped in that cell and cannot flow. The Fill tool modifies the elevation of the original DEM to eliminate these problems.
- At the top left of the Geoprocessing pane, click the Back button.
- In the search box, type "fill".
- Click the Fill tool.
- For ‘Input surface raster’, select the DEMRecon layer.
- For ‘Output surface raster’, rename the raster from “Fill_DEMReco1” to “FIL”.
- Ensure your Geoprocessing pane appears as shown below and click Run.
Calculating flow direction
Now you will calculate the cardinal direction in which water from each cell is expected to flow based on the elevations of surrounding cells. Each direction is assigned one of 8 integer values between 1 and 128 as indicated in the illustration below.
At the top left of the Geoprocessing pane, click the Back button.
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