Geography Reference
In-Depth Information
How many square miles? 28 __________. You may find the numbers that are presented a little
confusing. In the attribute table, count refers to the number of posts of a given height. In the
selection statistics window, count refers to the number of records being considered, while sum
refers to the total number of posts represented by those records.
47. Dismiss the Statistics window and the DEM attribute table. Zoom to the extent of the DEM. The
selected areas (bright blue) show those elevations between 901 and 1000 feet, which, as you
will note, is almost all the quadrangle. Clear selected features with the appropriate button on
the Tools toolbar.
48. Again experiment with various levels of magnification, including full extent of the DEM. Notice
that in most places the TIGER streams seem to flow down the valleys shown by the DEM, but
sometimes they don't. Again, GIS data is like other data: wide variations in quality—perhaps
even within a given dataset. Data sets from different sources are not always consistent. Pick
a stream and attempt to determine its direction of flow by looking at elevations. Some stream
designations in the southwest area of the map stop abruptly at the black line. Can you recall
why? ___________________________________ (Hint: Turn off cnty_bnd_spn arc.)
Comparing the DEM and the DRG
49. Compare the elevations “printed” on the DRG with those values given by the DEM: Zoom to
the bookmark River_Bend. Turn off the DOQ layer if it is on. Arrange the DRG and DEM layers
so that the DEM is at 'the bottom and the DRG is just above it. Turn both layers on. The DRG
image will obscure the DEM. Unhide the Identify window again and activate the tool if it is not
already. Set up “Identify from layer” as COLE_DEM. Use the Identify tool and MapTips on the
topographic quadrangle, pointing at contour lines where the text on the map shows elevations
(e.g., 600, 700, 850, and 900 feet, zooming out as necessary) near the bend in the river. You
visually see the elevations on the DRG, but the Identify results and MapTips come from the
DEM. Note the agreement (or, sometimes, lack thereof) between what is printed on the topo
map and the values of the underlying grid. Zoom out somewhat and try this exercise again. You
may notice better agreement between the two datasets as you get further away from the steep
slopes around the river. Auto-Hide the Identify Results Window.
50. Zoom to the extent of the DRG. Locate Lexington Reservoir No. 4 in the north central part of
the quad. Zoom in on it. By turning the DRG on and off, flip between viewing the DEM
and the DRG. Determine, from the DEM, the elevation of the surface of the water in the
reservoir. ___________ feet. According to the topo map, what is the elevation of the
spillway? ________ feet.
Contour Line Files
Another way of representing elevation on a map, familiar to you from your examination of the topographic
map, is through “isolines” or “contour lines.” For any given such line, every position on that line is
supposed to represent the same elevation. The contour lines you have seen so far have been “printed” on
the DRG, but a GIS can represent these lines as data as well.
28 There are 43,560 square feet in an acre. There are 640 acres in a square mile.
 
 
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