GPS for Dummies.pdf - Engineering Surveyor

42Part I: All About Digital MapsElevation dataYou’re likeliest to run into one of these two main data formats used to representelevation: Digital Elevation Model (DEM): The USGS uses this raster data format torecord elevation information (based on topographic maps) and createthree-dimensional representations of the terrain. Figure 2-13 shows amap of Mt. Bachelor, Oregon, generated from Digital Elevation Modeldata with the 3DEM mapping program. National Elevation Dataset (NED): This format shows digital elevationdata in shaded relief. It’s designed for seamless coverage of the UnitedStates in large raster files.Figure 2-13:A mapcreated withDEM data.Digital Raster Graphics (DRG)Digital Raster Graphics (DRG) data is a scanned image of a USGS topographicmap. These digital maps are available for free on the Internet or are sold commerciallyin collections on CDs or DVDs. Figure 2-14 shows a DRG map fromthe TerraServer-USA Web server. This map is a digital version of a 1:24,000scale topographic map.These digital maps are scanned at 250 dpi (dots per inch) and stored in aTIFF file format, using embedded GeoTIFF (geographic information) tags forlocation data.