3DEM is another program that the serious terrain modeler needs to have in their library of GIS applications.  You can download a the latest (free) version of 3DEM to experiment with.   You can also get an older version of the software called 3DEM70 for free. The older version is kind of crippled because it will not read the revised SDTS file format.  (The USGS, working through GeoCommunity for reasons that have to do with file compression made minor modification to the xxxx.cel0.ddf file format in the SDTS profile.  While the change was very much within the existing SDTS file specification, it immediately caused about half the SDTS readers in the world to become obsolete pending revision.  Since no one is maintaining older software like 3DEM70, users of these programs are out of luck.)  You can still get your files into 3DEM70 by using the utility  SDTS2DEM  to convert SDTS to  USGS DEM format. 

The best thing about 3DEM, and the reason that you need it is for its ability to overlay USGS topo maps over DEMS.  Recall in my section above on this technique I outlined how to do this using POVRay and Paintshop Pro.  The way described above is the hard way.  The great thing about 3DEM is its automatic registration of USGS DRGs to the corresponding USGS DEM.  In particular, the overlay of 1:24,000 topos over 1:24000 SDTS 7.5' 30m DEMS is pretty seamless and is the technique that I will demonstrate in this tutorial.  So here is how to do it:

The first thing that you need to do is go to gisdatadepot.com and download the 7.5' 30m SDTS DEM of your area of interest.  You do this by clicking on a state, then a county, then on 'Digital Elevation Models, 24K (for 1:24,000), and then on your quad.  Download the file. The USGS topo can be downloaded from a couple of free websites. Try www.topowest.com for starters. If your state is not offered at this site, see the information in the News section for more information.

Then back up one page and click on 'Digital Raster Graphics (DRG), 24K.  Go through the same exercise and select the corresponding USGS topo quad that matches up with the DEM you just selected.  Beware that these files are pretty big ( several megabytes) and will get huge (~40MB) when unzipped.

After you have both files downloaded and unzipped, open 3DEM.  Click <File>, <Digital Model>, <Terrain Model>.  Make sure that 'USGS SDTS DEM' is selected and load the DEM file.  You should see a familiar color depth DEM image.  Select <Operation>, <Apply Overlays> under the 'Terrain' window.  Select your topo map file and click <Load>.  3DEM will load the topo. (This step will take a few minutes as it processes the file). Once the file is loaded, select <Crop>.  A crop window will come up. Adjust the crop box to trim the map borders and press <Enter>.  After a few minutes, 3DEM will crop the border and register and overlay the topo map over the DEM. 

At this point, all  you have to do is set the 3DEM viewing parameters in order to view your overlay.  I redid the Salem, NY overlay that I described in the previous section, using the 3DEM utilities.  In fairness, the stunning qualify of the 3DEM overlay is directly related to the high quality of the DRG used in this tutorial.  However, making this overlay was a snap compared to the agony of doing it in Painshop.

3DEM also has utilities for manual registration of DRGs.  The process is obviously more involved than that procedure described here.  However, it gives 3DEM great flexibility in overlaying unmatched DRG data over DEMS.