GPS for Dummies.pdf - Engineering Surveyor

Chapter 16: Going Three Dimensional with 3DEM261Chapter 21 covers Web sites where you can download DEM files and other freemap data. The 3DEM online Help also has several sources for no-cost DEMdata. In your search for DEM data, be aware that elevation data can often befound in Spatial Data Transfer Standard (SDTS) format files. You’ll need thefree utility SDTS2DEM to convert SDTS data into DEM files that can be usedwith the programs discussed in this chapter. You can download SDTS2DEM atwww.cs.arizona.edu/topovista/sdts2dem.For this example, I use a DEM associated with the area around MountSt. Helens, the volcano in Washington State that erupted in 1980. Mount St.Helens is interesting because instead of blowing straight up, it blew outthe side. This (and a small dome in the center of the crater) makes the stillactive volcano a good candidate to view three-dimensionally.To follow along as I create a 3-D map, download the DEM file used in thisexample from the University of Washington’s Department of Geology Website. Here’s how:1. Go to http://duff.geology.washington.edu/data/watiles/index.html.2. Click the Hoquiam grid.3. Click the 10-meter DEMs link.Just a quick note about DEMs. You’ll often see references to 10-meterDEMs and 30-meter DEMs. These numbers refer to the elevation dataresolution: The smaller the number, the higher the resolution. For example,with a 10-meter DEM, the ground spacing between elevation pointsis 10 meters.4. Click the Mount Saint Helens grid on the map (in the lower-right corner).A Zip file (f2323.zip) downloads to your computer. The file is a bitover 2MB and contains a compressed version of the DEM file for MountSt. Helens.5. Uncompress the Zip file to access the DEM file.If you’re using Windows XP, double-click the Zip file to open it, and thencopy the f2323.dem file and paste it in a folder to uncompress and save it.If you’re not using Windows XP, you need a utility to uncompress the Zipfile and extract the DEM data. I like the free file archiver 7-Zip, which isavailable at www.7-zip.org.After you have a DEM file to work with, load it into 3DEM. This is simple:1. Run 3DEM.2. Choose Load Terrain Model from the File menu.3. In the DEM File Type dialog box, make sure that USGS DEM is selectedand then click OK.