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
(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 ...