Spatial Analysis Based on Raster Data Processing - Introducing Geographic Information Systems with ArcGIS - page 531

Now click the icon of Create Steepest Path on the 3D Analyst toolbar. Again click a cell. Note
that a line runs from that point downhill to a point of lower elevation. When you are through
being experimenting, use the Select Elements cursor to draw a large box around the entire map.
Press the Delete key to remove the contour lines and steepest path lines.
4. Find and run the Contour (Spatial Analyst) tool. From the drop-down menu of Input raster, select
ElevSurface. Make the contour interval 100, but use a base contour of some even hundred-
number (ends in 00) under the altitude minimum, say, 1500. For output features browse to
___IGIS-Arc_ YourInitials \Spatial_Analyst_Data\Hydrology_Data_SA
and name the feature ElevContours.shp. Click OK. Examine the contour lines added to the
map and the conformance with the surface ElevSurface. Label the contour lines with the field
CONTOUR. Zoom in on the hill top in the center of the map. Use Identify on two cells of the
surface (ElevSurface) somewhere on opposite sides of a contour line labeled 3500. What are
the values given by the raster? _______, _____.
5. Find the tool Slope (Spatial Analyst). Make the input surface ElevSurface. Make the output
measurement PERCENT_RISE. Note that the output will be a raster—call it ElevSlope—and put
it into Hydro.gdb. Click OK. The Slope raster will cover up the ElevSurface raster. Notice that the
slope is greatest (like about 60 percent—100 percent is 45 degrees) where the contour lines
are closest together.
6. Find the tool Aspect (Spatial Analyst). Make the input surface ElevSurface and the output
ElevAspect. Again, a raster is placed on the map that covers up the previous ones. As you
know, aspect is the direction the slope faces. Zoom to full extent. What you see is something of
a hodgepodge—a riot of colors. Examine it further in the next step.
7. On the Spatial Analyst toolbar, set the drop-down menu to ElevAspect. Click the Histogram
button. Comparing the colors of the histogram with those of the T/C for Aspect of ElevSurface,
in what direction would you say most of the slopes faced? ________.
8. Find and run the Hillshade (Spatial Analyst) tool. Make the input surface ElevSurface and call
the Output ElevHillshade. (You are putting these created rasters in the Hydro.gdb file database,
aren't you? Sorry to nag.) Click on the Azimuth and Altitude fields to understand what is meant
by these terms. Accept the default setings of 315 and 45. Click OK. Turn off all layers except
ElevSurface and ElevHillshade. Go to full extent. By flipping the Hillshade layer off and on, you
can get an idea of what the area would look like with sun in the northwest, 45 degrees above
the horizon, and, from ElevSurface, why it would look that way.
9. Generate a flow direction raster: Find and start the Flow Direction tool. Use ElevSurface as the
Input and FlowDir as the Output flow direction raster. Set the option that does not to force water
off the edge of the raster. (Click the Output Drop Raster text box and read about it. Create such
a raster, called ElevDrop.) Click OK.
10. FlowDir and ElevDrop will both be added to the map. From the T/C you can determine the
maximum and minimum slopes based on ElevDrop. What are they? ________%, ________%.
Remove ElevDrop from the T/C.