Spatial Analysis Based on Raster Data Processing - Introducing Geographic Information Systems with ArcGIS - page 535

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. Call the result SuspectAreas, and make sure it goes into Hydro.gdb. Move
pollution_pts above SuspectAreas in the T/C. Open the attribute table of SuspectAreas. How
many cells might be involved in the search for the location that is polluting 550? ________.
675? ________. This tool could help trace sources of pollution.
24. Using Raster to Polygon (in Conversion Tools) make a polygon shapefile of UniqueBasins. Call it
Basins_Outline.shp and put it into the Hydrogology_Data_SA folder. Make its color Hollow and
the width of its outline 3 picas, in black. Turn off all layers except Basins_Outline and FlowDir2.
With Basins_Outline at the top of the T/C, note that all flows are either away from boundaries or
parallel to boundaries of the basins (look again at Figure 8-21).
The distinction between basins and watersheds is a subtle one. Also, they are not completely distinct.
In terms of ArcMap, basins are determined only by flow direction, which is determined by the elevation
surface, That is, topography is the primary ingredient in basin determination. The water shed calculation is
based on the aggregation of cells that feed a particular point, usually on a stream.
25. Save the map as Boundaries & Streams.mxd in the folder Hydrology_Data_SA. Make a layout
and print a map that consists of: Basins_Outline (with dark outlines of hollow polygon basins)
StrmOrderStM (with a symbology that goes from Cyan-Light to Blue-Dark). Note how streams,
being confined to basins, become larger as they move toward the points at which they finally
flow off the edge of the map.
26. Close ArcMap.
Exercise 8-8 (Review)
Checking, Updating, and Organizing Your Fast Facts File
The Fast Facts File that you are developing should contain references to items in the following
checklist. The checklist represents the abilities to use the software you should have upon completing
Chapter 8.