Geography Reference
In-Depth Information
Exercise 2-4 (Major Project)
A Look at Raster Data
Here's a dichotomy: Be aware of the difference between (1) opening a map and (2) adding data to a map.
To open a map, you use File > Open to get to an Open window, which then allows you to browse for an
MXD file. On the other hand, if a map is already open, you use Add Data (which you can get to a variety of
ways) to put an additional dataset on the map.
1.
Start ArcMap. If you have not done anything else in ArcMap since completing Exercise 2-3,
River_Map_2.mxd should conveniently available for selection in the Existing Map section of the
Getting Started window. Highlight it and click Open. (If River_Map_2.mxd is not available, be
sure Existing Maps > Recent is selected on the left hand side of the Getting started window. If
River_Map_2.mxd still isn't available, select Existing Maps > Browse for more and navigate to
the location of River_Map_2.mxd. Click Open.)
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Digital Raster Graphics and Cell-Based Files
Shapefiles, coverages, and geodatasets are all capable of storing points, lines, and polygons. These three
are illustrative of the data model we have called “vector.” Now we turn to a completely different method of
representing geographic data: “raster.”
Digital raster graphics (DRG) files are images (photographs, pictures) of the 7.5-minute topographic
quadrangles (topo maps) produced by the United States Geological Survey (USGS). They have been
scanned into a graphic image format (TIFF—Tagged Image File Format) and provided with information that
fixes them in geographical space, making them GeoTIFFs.
2.
Display a layer based on the digital raster graphic (DRG) image of the Coletown quadrangle:
Set the T/C to List By Drawing Order. Remove the layer CNTY_BND_SPN
polygon
. Zoom to the
GPS track. In the T/C, right-click Layers. Add COLE_DRG.TIF from
[___]IGIS-Arc\River.
(If asked, do not build pyramids but read the explanation.) Zoom the view to the full extent of
the DRG to see what a USGS DRG image looks like. A part of the city of Lexington is in the
northwest corner; the river datasets of interest are in the southeast. Zoom back to the area of
the GPS track and notice that only part of the GPS track is in the area covered by the Coletown
quadrangle. (See Figure 2-20.)
When (and only when) List By Drawing Order is selected, you may rearrange the order in which
entries appear in the T/C. This is important because the T/C order determines the order in
which the layers are drawn on the computer monitor. To change the order, you simply drag a
layer's name to where you want it to be in the T/C. In general, you should put point-based layers
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When ArcMap is already running you can open a map by (a) pressing Ctrl-O, (b) Choosing File > Open, or (c) click-
ing the Open An Existing Map (see Status Bar or ToolTip) icon on the Standard toolbar. Opening a map requires that
you do something (either save or discard) with the map that is presently displayed.
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