Geography Reference
In-Depth Information
FIGURE 2-36
If you try to open an attributes table of a TIN dataset, you will discover that it has none. In most
geographic datasets the values produced by the Identify tool come from an attribute table. In the TIN,
however, the values placed in the Identify Results window are calculated on the fly immediately—based on
where you have placed the cursor and clicked.
6. Collapse the legend for the TIN in the T/C. Zoom to the GPS track, and pan to fill the
window with TIN. Again, with the MapTips of the DEM active, compare the DEM elevations with
some TIN elevations as revealed by the Identify tool. Under the Display tab in the Properties
window for the TIN check Show MapTips, which will take the MapTips away from the DEM and
assign them to the TIN. Slide the cursor around the display looking at elevations, created by the
TIN MapTips. Click on a few points to verify that the MapTips agree with the elevation shown by
Identify.
7. Change the Primary Display Field of the TIN (in Layer Properties, under the Fields tab) so that
MapTips shows Slope. Click Apply, then OK. Experiment by sliding the Select Elements cursor
around on the map. See Figure 2-37. Near the river you can find very steep slopes, since the
river basically runs through a canyon it has cut into the rock. What is the greatest slope you
can find near the river? _______ degrees. The slope you see is expressed in degrees measured
from the horizontal plane. 32
32 You have to be careful when dealing with the numbers that specify slope. Sometimes slope is expressed as a
percentage: the rise in elevation divided by the run along the surface of the Earth, times 100 to make it a percentage.
If you want the angle of a given percent slope, divide the percent slope by 100, and find the angle whose tangent is
that value. Such an angle is called an arctangent, defined as “the angle whose tangent is x.”
 
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