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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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Preparing to Create a Cost Surface
You have just made the raster containing source cells. The next step is to create a cost surface raster that represents “resistance to travel” by assigning travel costs to every cell in the study area. In those cells where there are roads, you will base the travel cost on speed limits. In areas where there are no roads, you will simply assign a high cost based on a very low speed limit (5 miles per hour), primarily to keep cars on the roads. It would be reasonable to ask, “Why not just assign a super-large impedance to off-road cells?” You won’t do this, because you want to allow every cell in the study area the possibility of access. If you assign an impossibly high impedance value to cells that don’t have roads in them, you exclude those cells from consideration for the park because they could not be accessed unless a road ran through them. Presumably, if a group of cells is to become a park, a low-speed access road would be built. You do, however, want to completely exclude freeways to force travel on local roads and to prevent a path from crossing the freeways. In the following steps, you will select highways, county roads, and local roads to make a raster based on their speed limit values. You will then reclassify the nonroad cells to give them a speed limit of 5 mph.
____ 13. Select the proper roads and create the Speeds raster: The Speed limits data will be a raster representation of the selected roads; the values in the cells are the ...

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