Geography Reference
In-Depth Information
4.
Set Environment Variables: Geoprocessing > Environments. Leave the coordinate system
undefined. Make the Processing Extent Same as Layer “STUDY_AREA.shp.” Under Raster
Analysis Settings, set the cell size to 150 (under As Specified Below). Click OK. Resave the
mapfile Site_Park.mxd, in case you have to come back to this point.
Now that setup is complete and you begin thinking about the problem, a couple of issues might come
to mind.
(a) How are you going to identify the off ramps?
(b) How are you going to exclude cells that have no roads, and cannot be traversed? Recall that
your previous experience has been with rasters in which any cell could be traversed.
As to (a), you will be able to select the ramps based on values in the Roads table, which indicate the road
types. For (b), once you have identified cells with road segments, you can set their impedance based on
speed limits, which will also be available in the Roads table. For the cells without roads, you can set a
high impedance, or cost, so that it would be prohibitive to drive through very many of them.
5.
Open the table containing attributes of Roads. Some of the data massaging has already
been done for you. The roads have been clipped to the study area, and the number of roads
has been reduced to 12 sizable networks (from 29,326 road segments in the valley). The
12 road networks were derived by merging similar road types based on their Census Feature
Classification Codes (CFCCs). (Note: Roads.shp is a feature-based layer, not a raster dataset; it
will be in the Park_Data folder, not the Redlands geodatabase.)
The assumption has been made that all roads of the same CFCC will have the same speed
limit. (The speed limit data is in a table that you will later join to the Attributes of Roads table).
6.
Add the CFCC_Speeds.dbf table to the map:
Open it. Notice the different road classifications
and the speed limits associated with each.
7.
Join the two tables:
Right-click ROADS in the T/C. Choose Joins and Relates. Click Join. In the
Join Data window, you want to use the “Join attributes from a table” selection. The field that
the join will be based on is CFCC in both layers (that is, window items 1 and 3). The table to
join (item 2) is CFCC.dbf. Click OK. The data contained in CFCC.dbf is added to the Attributes of
Roads table. Close CFCC.dbf and open the now-enhanced Attributes of Roads table and scroll
or widen the window so that you can see all its fields.
The Roads table identifies the Off_ramps with a code (A63). You want to make a raster containing
source cells that represent road segments with the A63 designation; you want to assign No Data to all
other cells. You will select the Off_ramps and then make a raster dataset of the selected features.
8.
Select the Off_Ramps:
With the Attributes of Roads table active, select the record containing
Off_ramps
.
Arrange things so that you can see the Off_ramps selected in the map, highlighted
in cyan.
9.
Create the source raster:
Find and start the Feature To Raster tool. The Input Features should
be ROADS. Make the Field ROADS.CFCC. (The field chosen here is only informational; the
integer value of the one record in the raster will be 1.) If necessary, set the Cell Size by typing
150 (recall that the units are feet). To create the name, browse to
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