The Third Spatial Dimension - Introducing Geographic Information Systems with ArcGIS - page 566

with elevations differences greater than or equal to 5 feet shown, and elevation differences of
less than that not shown. Even less detail will be evident between 1:5000 and 1:10000. The
same holds for differences between 1:10000 and 1:20000. The least detail, with only elevation
differences of 40 feet or greater, will be shown when zoomed out beyond 20000. In all, how
many levels of detail are specified here? ______
85. In the New Terrain window that lets you specify pyramid properties, press Add four times.
Change the numbers so that the window looks like Figure 9-14. Click Next.
86. Review the window of terrain settings. If they are not correct, go back and fix things up. If they
are, press Finish. You will be informed that the Terrain has been created but not built. Build it,
and be prepared to wait a bit.
87. Once the terrain (My_Terrain) is built, use ArcCatalog to look at it with different zoom levels,
starting with Full Extent. Note the difference in the amount of detail that is apparent, which is
illustrated by being able to see more triangles as the scale increases.
88. Start ArcMap with a Blank map. Dismiss ArcCatalog. Add My_Terrain. What does the scale
textbox in the standard tool bar say? _________ Type 5001 into that box and press Enter.
Observe the result. Now type 4999 into the scale box (which gives you almost precisely the
same scale), but carefully watch what happens to the map immediately after your press enter.
What you are seeing is the software recalculating the TIN to the specifications you gave earlier.
That is, you see differences of 5 feet, whereas before you saw 10 foot differences.
89. Investigate the differences using scales of 1:2501 and 1:2499. Any scale greater than 1:2499
will give you all the detail the data set is capable of, which is a lot!
90. Zoom in more and use the Identify tool on a triangle. What is the elevation at the centroid of
the triangle? ________. What is the slope? ________. What is the aspect? _______.
91. Because of the shading of the TIN that ArcGIS provides you can probably estimate the aspect of
a triangle (i.e., the compass direction it faces). Pick one whose direction seems obvious. What
do you guess it to be? _______ degrees (north being 360). What is the aspect as indicated by
the Identify tool? _______ What are the elevations of the triangle with the cursor placed close to
each of the three apexes. _______, _______, and _______. Close ArcMap.
As you can see, building an Esri Terrain is not a trivial matter, but LIDAR and the Terrain bring us
closer to the (ultimately unrealizable) goal of a perfect model of the Earth's surface.