GPS for Dummies.pdf - Engineering Surveyor

Chapter 16: Going Three Dimensional with 3DEM269Three files are inside the Zip file:• o46122b2.tfw: Each DRG has an accompanying World file thatcontains georeferenced information. The World file has the samefilename as the DRG but typically ends with a .tfw extension.• o46122b2.tfw83: Another georeferenced data file, this one containsinformation for using the NAD 83 datum with the map (which isclose to WGS 84) instead of the NAD 27 datum typically used withUSGS topographic maps. (See Chapter 2 to read about the differencesbetween different map datums.)• o46122b2.tif: This is the actual DRG map image file.3DEM uses just the TIFF map image file; you don’t need to worryabout the other two files.The TFW files that come with DRGs usually contain redundant georeferenceinformation; the same data is embedded in the map image file as a GeoTIFFtag. Depending on the software you’re using, the data in the TFW file mighttake precedence over the embedded data in the image file for georeferencingthe map image.Loading the DRG as an overlayAfter you uncompress the DRG file, you can load it into 3DEM. Make sure youhave the DEM data loaded first, and then do the following:1. Choose Apply Map Overlay from the Operation menu.The Apply Map Overlay dialog box is displayed.2. Click the Load button.A file Open dialog box appears.3. Select the DRG file you want to overlay (in this case, o46122b2.tif),and then click Open.A message appears asking you to wait while the data is loaded and isprepared for overlaying. When the process is finished, the message GeoReference Complete appears.4. Click the Accept button.3DEM renders the overlay image; a status message appears during overlaygeneration telling you how many lines are left to process.When 3DEM is finished, the overlay map is displayed.Instead of showing only a relatively boring terrain elevation map, thetopographic map is merged with the DEM data to create a shaded reliefmap that shows labeled features, contour lines, map grids, and a graphicrepresentation of elevation (as shown in Figure 16-6).