Geography Reference
In-Depth Information
Let's now make a raster in which the speed in each cell is the same as the speed limit in that cell, except
for Interstate cells (which we will make low because we don't want our path to the park to try to cross an
interstate). The No Data cells we will set to 5 mph. To do all this, we will use the Reclassify tool.
14.
Make a raster with speeds in the cells:
Find and start the Reclassify(Spatial Analyst) tool.
The Input raster should be SpeedLimits. Press the Unique button to be sure that you get all
the different values of the cells. The Old Values are 1 (for private roads), 30, 45, 50, 65, and
No Data. The suggested new values are 0, 1, 2, 3, 4, and No Data. You do not want these
suggested values. Mostly, you want the new values to be the same as the old values, except for
the No Data value, which you want to be 5, and the Interstate value (65), which we'll make 2.
Type in the new values: 1, 30, 45, 50, 2, and 5. Browse and call the new Output raster Speeds
(in Redlands.gdb, of course). Click Save, then OK.
15.
Using Identify, examine the cells containing the speeds by panning and zooming:
Use the
Identify tool (operating on the Speeds raster) to verify that the data cells that were formerly
NoData cells have a value of 5 and that other cells have appropriate values. Turn off the Speed_
Limit, Off_Ramps, and the Study_Area entries, if they are on. Move the Roads feature class to
the top of the T/C and make sure it is on. Turn on the Speeds raster. Next, zoom in on an area
that includes part of the interstate where the ramps are. Notice how the speed limit cells follow
along with the local roads. Notice also that all the cells between the roads now have the 5 mph
value. Zoom in some more to see the relationship between the feature dataset Roads and the
raster dataset Speeds. When you are finished looking, choose Full Extent from the map menu.
16.
Examine the Properties of the Speeds raster:
In the T/C right-click the Speeds layer name and
click Properties > Source. How many columns rows are there? ________. Rows? ________.
What is the uncompressed size of the raster? ________. Dismiss the Layer Properties window.
Building a Cost Surface
Now you have the raw material (those speeds at which cars going to the park may travel) for building a
cost surface. What numbers do you put in the cost surface cells? You cannot put miles per hour (mph),
because then higher speeds would imply greater resistance to flow—the opposite of what you want. You
could use the reciprocal, hours per mile. To get this reciprocal, you simply divide “1” by miles per hour,
which gives you hours per mile. For example, 50 mph is 0.02 hours per mile; 25 mph is 0.04 hours per
mile. Since 0.04 is a larger amount than 0.02, and that larger amount represents a slower speed, 0.04 is
what you want to see in your cost raster.
But there is another consideration: It is important to express the cell costs in terms of the units of
the raster and the problem statement. You are interested in drive times in minutes. And the raster is
measured in feet. The numbers you have been given to work with are miles and hours, but what you want,
strange as it may seem, is minutes per foot! How do you convert hours per mile to minutes per foot?
Multiply hours per mile by a conversion factor: 0.0113636, which is the number of minutes it takes to
travel a foot at the rate of one hour per mile.
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One hour per mile is 60 minutes per 5280 feet. 60/5280 equals 0.0113636—your conversion factor. So, for example,
50 miles per hour is 1/50 hours per mile. 1/50 * 0.0113636 5 = 0.000227272 minutes per foot. If a cell is 150 feet across,
say, from east to west, it therefore takes 150 * 0.00022727 or 0.034 minutes to cross it at 50 mph. (Or you can just take
my word for it.)
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