Geography Reference
In-Depth Information
Improving the Understandability of the Map
You can get a better idea of travel times by redoing the legend of Drive Time. The current legend simply
breaks the drive times up into equal intervals, resulting in unwieldy ranges. A better set of classes can be
defined, and a better legend can be created.
21.
Through Drive_Time Properties, bring up Symbology. In the Show box, select Classified. Specify
10 classes, then press Classify. In the Classification window, pick Manual for the Classification
Method and make the break values 3, 5, 10, 15, 20, 25, 30, 40, 50, and 60. Click OK. Back in
Layer Properties pick a color ramp you like. In the Label column, type in the top value of each
range: 3, 5, and so on. Click Apply, then OK. Look again at the T/C entry for Drive_Time. Better?
The new legend uses more appropriate and readable divisions of driving time.
22.
Close all legends except ROADS, Off_ramps, and Drive_Time. Clear any selected features. Make
the symbol for Off_ramps and ROADS solid black. From the top to the T/C, order the layers: ROADS,
Off_ramps, and Drive_Time. Turn them on and everything else off. Explore the resulting map, using
Identify on the Drive_Time raster. Click in a number of cells—both at full extent and zoomed in.
It should be apparent that the mathematical massaging you did earlier (calculating minutes per foot) to
make the ultimate results show up as minutes was worthwhile.
23.
Use File > Save As to save the project with a new name: Time_to_Park_Sites_
YourInitials
.mxd.
Exit ArcMap.
Exercise 8-7 (Project)
Watershed Analysis
To review from the Overview and prepare you for the next exercise, let's recap the various steps in
determining a model of surface water flow that you encountered in the Overview of this Chapter. You might
want to quickly review what was said there.
It starts with elevation information, since water (unless extremely provoked) runs downhill. In this exercise
the elevation layer is called Elevpts. From Elevpts we will create ElevSurface. From ElevSurface we will
create FlowDir, which will define, for each cell in the raster, the adjacent cell into which the water will flow.
We can also create a raster that shows the maximum downhill slope for each cell, called ElevDrop.
From FlowDir we can determine three other rasters:
UniqueBasins—A basin is an area where the elevations and slopes are such that the ultimate
drainage all winds up in the same place
❏
FlowAccu—A measure of how much water arrives in each cell, given that every cell initially gets one
unit of water
❏
AnySinks—Showing any areas inside the study area where water could not drain to an adjacent cell
❏
From FlowAcc we can determine StrmChannels by saying that if a cell has more than a prescribed amount
of water in it, it is part of a stream. StrmChannels lets us produce two new rasters:
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