(a DEM Helpből)

"3DEM allows you to overlay surface maps showing roads, cities, lakes and rivers, and other features over the 3D terrain image. For USGS 7.5 minute DEMs, these overlays can be Digital Raster Graphic (DRG) or Digital Orthophoto Quad (DOQ) files. These map files can be very large (50 to 100 Mbytes), and processing of these overlays can be a very lengthy process. Processing of map overlays is not recommended if you have less than 256 Mbytes of RAM or less than a 500 MHz processor.

Overlay maps must be fixed to the geographic coordinates of the underlying DEM. To apply a map overlay using the Map Overlay Dialog Box, take the following steps in sequence.

STEP 1 - Locate a source of DEMs and corresponding surface maps that cover the same geographic area at about the same scale. For the United States, good matches are the 7.5 Minute SDTS DEMs and matching DRGs provided by the USGS at http://www.gisdatadepot.com/dem/. The following instructions will illustrate overlay of Digital Raster Graphics on a Digital Elevation Model using the Snowshoe Peak, Montana, DEM and matching DRG available at this location.

STEP 2 - Download the Showshoe Peak SDTS DEM and matching DRG from http://www.gisdatadepot.com/dem/. These files can be found under Lincoln County, Montana. The DEM will be downloaded as a tar.gz file from “Digital Elevation Models (DEM) 24 k.” The DRG will be downloaded as a zip file from “Digital Raster Graphics (DRG) - 24k.”

STEP 3 - Decompress and extract the DRG graphics file from the downloaded zip file. Winzip can be used for this operation. The result will be a DRG geotiff graphics file (*.tif).

STEP 4 - Start the 3DEM program and Load the Digital Elevation Model. Choose “File - Load Terrain Model” from the Overhead View menu, and choose “USGS DEM” as the File Type. Then locate and load the DEM tar.gz file from Step 2 above.

STEP 5 - Load the DRG as a map overlay from the Overhead View before creating the 3D Scene. Choose “Operation - Apply Map Overlay” (or Function Key F5) to bring up the Map Overlay Dialog Box. Click the “Load” button and select the DRG file from Step 3 above. 3DEM will read the coordinates of the DRG geotiff file, convert it into a geo-referenced overlay, and apply it to the surface of the Digital Elevation Model. The result will be previewed in the Map Overlay Dialog Box as shown below."