GPS for Dummies.pdf - Engineering Surveyor

268Part III: Digital Mapping on Your Computerfor publication purposes, this option is useful for saving a large imagethat can be reduced in size yet still retains detail. Print Scene Image: Prints the image. Save Rotation Animation: Rotates the 3-D image 360 degrees and savesthe output as an AVI file that can be replayed as a movie with WindowsMedia Player or other multimedia software.Overlaying Digital Raster Graphics DataUsing DEM data to create a three-dimensional map is pretty cool, but 3DEMallows you take one step beyond simple terrain modeling. You can also take aUSGS topographic map and overlay it on top of the DEM data to create a 3-Dtopographic map.To do this, you need a topographic map in Digital Raster Graphics (DRG)format, which is another map data type produced by the USGS. A DRG fileis a scanned version of a topographic map that has georeferenced data associatedwith it. DRG maps typically come in the TIFF graphics format.Downloading a sample DRGIf you’ve read this chapter to this point, continue with the running example.Download the DRG that’s associated with the Mount St. Helens DEM file alreadyloaded into 3DEM. (See the earlier section, “Using DEM Data to Create a Map.”)To download the DRG1. Go to http://duff.geology.washington.edu/data/watiles/index.html.2. Click the Hoquiam grid.3. Click the clipped Digital Raster Graphics link.The other link at the Web site, Digital Raster Graphics, contains thecomplete (unclipped) 7.5 minute topographic map, including the collar(the white space border that surrounds the map). 3DEM can remove aDRG file’s collar so it isn’t displayed in your map; if a clipped version isavailable, it saves a bit of processing time.4. Click the Mount Saint Helens grid on the map (in the lower-rightcorner).The file o46122b2.zip downloads to your computer. The file is a bit over3MB and contains a compressed version of the DRG for Mount St. Helens.5. Uncompress the Zip file to access the DRG data.