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Introducing Geographic Information Systems with ArcGIS: A Workbook Approach to Learning GIS, 3rd Edition by Michael D. Kennedy

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Finding Pollution Culprits
Suppose there are monitoring stations on streams, looking for dangerous substances in the water. If a station reports a problem we would like to know the location of the source of the pollution. Since each of the streams has associated with it a watershed we may narrow the search, using the WaterShed tool, which finds all the upstream cells that flow down to a given point—the point where pollution is discovered.
Specifically, the WaterShed request finds the up-gradient cells of a specific set of cells; those cells may or may not be cells in stream segments. The POLLUTION_PTS dataset (points at which pollution has been found) indicates pollution in streams at two points.
____ 22. From
___IGIS-Arc_YourInititalsHere\Spatial_Analyst_Data\Hydrology_Data_SA
add as data the raster pollution_pts. Turn off all other layers. In the pollution_pts attribute table, select both records so that you have a better chance of spotting the points, which are single pixels, on the map. If that doesn’t work, turn the symbols of the two cells of the pollution_pts raster (550 and 675) bright red. (If that fails use the Raster to Point tool (in Conversion Tools) to make a points shapefile of the two raster cells, and make the points large enough so you can’t miss them.)
____ 23. Use the Watershed tool to determine those areas that drain to the points. The Input flow direction raster is FlowDir2. The “pour point data” comes from pollution_pts. Make the Pour Point field Value. ...

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