Geography Reference
In-Depth Information
21.
Add a layer from an orthophoto “TIF” file:
Zoom to the GPS track. Navigate to [___]IGIS-Arc\
River and add a layer to the map based on COLE_DOQ.TIF. (Ignore any warnings about projecting
layers. If asked, do not build pyramids.) If necessary, drag the COLE_DOQ title next to the
bottom of the T/C (with only the DRG layer below it). Zoom to the extent of the orthophoto
image layer. See Figure 2-27. You can get an interesting perspective on the images by flipping
COLE_DOQ.TIF off and on. Notice the water filtration plant and other artifacts on both the DRG
and the DOQ. Zoom up on the area of the island. Observe that you see the island on the DRG
(and see it outlined by vectors) but on the DOQ the island has disappeared. It may well have
been under water at the time this photo was taken. A 20-foot rise in water level on this river
during floods is not uncommon.
FIGURE 2-27
22.
Experiment with different levels of magnification on the DOQ. Notice that when you are zoomed
in very tightly, the “meaning” contained in the image “disappears” in that you see only squares
(pixels) of various colors. Zoom so that there are about 10 pixels across the screen. How many
feet across is each square? ____________. What length is this in meters?
26
_________. This
is the resolution of the dataset. Crudely, the resolution is a measure of the limit of how much
detail you can wring out of the data. Zoom back to the extent of the DOQ.
26
There are 39.37 inches in a meter.
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