Geography Reference
In-Depth Information
Developing a Raster with Cost Distance
The following steps will illustrate the principle of calculating cost that is based on both distance and a
cost surface. First, a quick review question:
16.
What would be the cost of the path going horizontally through a cell (cell size 10, cell cost
0.85)? Answer: 8.5. How about the cost of moving through that same cell, going diagonally?
________.
17.
Turn off all visible entries except Square_Grid. In the steps that follow, keep Square_Grid at the
top of the T/C to delineate the cells of the displayed rasters.
The Tolls dataset is an artificial cost surface raster dataset. The surface is divided into rows of cells; all
the cells in a given row have the same value.
18.
Add the cost surface raster, Tolls,
8
from
___IGIS-Arc_
YourInitials
\Spatial_Analyst_Data\Proximity_Data_SA\Prox_Exp.gdb
In Layer Properties, under the Symbology tab, click Stretched under Show. Right-click the color
ramp and turn off Graphic View. Make the color ramp of Tolls “Purple Bright”. Press the Labeling
button. Type in ”9” for Number of Intervals and press Generate. Press OK. Press OK in Layer
Properties. Look at the T/C for the values of the rows. See Figure 8-49 for the image.
Imagine that the center of each cell contains a toll booth. Depending on which row the cell is in, the
toll booth charge varies from 25 cents to $1.15 to pass through
one unit of distance
of the cell. In the
Tolls raster, all the cells in the bottom row have values of 0.25; the top row cell values are 1.15. So, to
cross a single cell in the top row in an east-west direction would cost $11.50 (1.15 dollars times 10
distance units). (Since the cost is based on each unit of distance, it costs more to cross the cell in a
diagonal direction because the distance is longer.) Use the Identify tool to examine the values of cells
in the rows.
19.
Examine the source raster:
In the T/C move Onecell to just under the square grid and turn it
on. Recall that Onecell was the source raster you originally worked with that contained a single
source cell. Turn off the tolls raster.
20.
Determine cost weighted distance using the Cost Distance tool
:
Where is it? ________________
_________________________ Start the tool. The Input raster should be Onecell. The Input
Cost Raster should be Tolls. Call the Output Raster TCostOnecell (the total cost of the path
from the center of each cell to the center of the source cell in the raster Onecell). Click OK.
When the new raster, TCostOnecell, arrives in the T/C, doubleclick the name to bring up its
Layer Properties. Click Symbology. Find the Invert box and click it on. Click Apply. Click OK.
21.
Arrange the top entries (Square_Grid, Onecell, TCostOneCell) and look at the new legend. Each
cell contains the cost of moving across the cost surface raster from that cell to the source cell.
You can get a general idea of the relative costs by looking at the “colors” of the raster.
8
If you are using version 10.0 and you see only a gray square on the map, do the following: Right click Tolls, the
Properties > Symbology > Show: Stretched. Then Stretch Type: Standard Deviations. Say Yes to Do you want to com-
pute statistics. OK. You should now see gray stripes for Tolls.
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