Geography Reference
In-Depth Information
because Pythagoras tells us that the sum of the squares of the two sides of a right triangle
equals the square of the hypotenuse (900 plus 1600 equals 2500, whose square root is 50).
9.
Verify distances using the Measure tool
:
Look at the Measure tool icon. You may be
disappointed to find that it is “grayed out.” This disappointment is caused because we haven't
defined units for the raster. You know how to solve this problem: Data Frame Properties, and
so on. Use kilometers for both Map and Display units. Once Measure is operating, click a cell
and use the Identify tool to obtain its value. Now click the Measure tool, fixed up to display
in kilometers, and verify that the distance from the center of your chosen cell to Onecell is
approximately the same as the value displayed in the Identify Results window.
10.
Determine the distance from a source cell to a diagonal neighbor:
Use the Identify tool to
check the distance from the source cell to the cell in the lower-left corner of the raster. What is
it? ____________. Use the measure tool to determine the distance from the center of the lower
left cell to the center of OneCell. ____________.
The distance should be 14.142136, which is approximately 10 times the square root of 2. (10
2
+
10
2
=
200; the square root of 200 is approximately 14.142136.)
11.
In Customize > Toolbars find the Spatial Analyst toolbar and turn it on. In the drop-down menu
select MapDist1, then click the Histogram button at the right of the toolbar. (Enlarge that
window if necessary.) Looking at the T/C (for the “colors” of the vertical bars) and using the
Identify tool on the histogram (right-click on vertical bars in the bar graph), how many cells are
about 30 units away from Onecell? ____________. Dismiss the Histogram, the Measure window,
and the Identify Results window.
To this basic understanding of Euclidean distance in rasters, we can add some capabilities. The new
features that you will work with are: (a) more than one source cell in the raster and (b) a cap, or cutoff
value, so that no value greater than the cap will be recorded in any cell.
Examining Many Source Cells and the Capping Distance
12.
In the T/C turn off Onecell and MapDist1. Add the raster Manycells to the map. Notice the
presence of several cells containing data, while the rest of the raster shows No Data. There is still
a single source cell in the southwestern portion of the raster, but there is also a line of source cells
in the northeast and a clump (cluster) of source cells in the northwest. What are the values of the
three zones? ________, ________, ________. Change the colors of all the source cells to black.
You will generate a raster containing distances to the nearest source cells, provided that the distance
is no further than 30 kilometers. You can limit the search for the closest source cell so that it does not
exceed a given value, thereby excluding cells in the output raster that are more than a certain distance
away from any source cell. (Those cells will contain NO DATA.) You are telling the software “if you have to
look further than this to get to a source cell, just forget it.” Such a number may be called a limit, a cap, a
cutoff value, or a threshold.
13.
Use the same Spatial Analyst tool (Euclidean Distance) as you did in the previous steps to
create the new distance raster from Manycells. Browse to make its name DistManycells. In the
text box for Maximum distance, type 30. The Output cell size should be 10. Click OK.
Search WWH ::
Custom Search