I received an email a couple of months ago from Paul Pingrey who sent me an interesting DEM overlay. The map was quite striking and the overlay image was not immediately recognizable. He told me that he used a technique that used USGS (and other) DLG data, dlgvPro (now called Global Mapper ) and 3DEM . He gave me permission to pass his clever technique along, so here it is, the patented Paul Pingrey overlay technique.

The folks at Global Mapper were kind enough to lend me a copy of their program for a week and I took advantage of it to try Paul's technique. (Click on the link to download a free demo version with the export features disabled if you want to follow along with this tutorial.)

First, you start with a DEM, preferably a 1:24,000 DEM downloaded from gisdatadepot.com . I chose my usual Salem, NY subject. While you are there downloading the DEM, download a Transportation DLG. I used the 100K package because the 1:24,000 Transportation DLG for Salem was not available. There was also a Tiger Census package that I did not try either. Global Mapper can ingest a wide variety of DLG data so you can experiment to find out what works best.

After downloading and unzipping, you will have to load the DLG package into Global Mapper and see what you have got. You do this by selecting 'Open as New' or 'Open into Current' depending on whether it is the first or subsequent files and then selecting the proper file type, in my case 'DLG-O' files. There were several files for the greater Glens Falls area. I chose the files outlining roads, trails and railroads. There were three files altogether that coincided with the Salem quad and I loaded all of them.

Next I loaded the DEM. Global Mapper overlayed all of the layers over the Atlas Shaded DEM. I then turned on the contour line option by selecting 'File' 'Generate Contours' and set the spacing to 20 meters. Global Mapper automatically generates a shaded relief overlay map. Now this is where the trick comes in. Global Mapper does not have the capability to position the viewing perspective arbitrarily. It remains fixed directly overhead. In order to achieve the arbitrary perspective, Paul turned the Global Mapper image into a 3DEM overlay. This is how it is done.

First, select 'Tools', 'Configure', Vertical Options'. Disable the hill shader and set the Vertical Exaggeration slider to zero. You should now have a "flat" map with no shading like the one shown in the first two figures to the right. Use the zoom tool to position the map correctly on the screen. Since we are going to do a screen capture we want it to be as large as possible without being clipped by the edge of the screen. Switch the coordinates to decimal latitude and longitude. Using the cursor, obtain and carefully note two diagonal corner coordinates. Now select 'File', 'Capture Screen Contents to Image'. Output the screen as a BMP file.

Next I entered Paintshop Pro and opened a blank (black) window 2000 X 2000 pixels. I opened my BMP and added it to the blank window as a new layer. I then merged the layers. This had the effect of matting the BMP against a stark black background. At this point, I could have added text to the map, for example the name of the town, local terrain features, etc. I saved the image as a BMP.

Now open 3DEM and load the DEM. Load the overlay image as described in previous sections. The only difference is that we will not crop the image, but will leave the black border in place. However, we will place the red and green markers on the corners of the image, not the corners of the matte. Render with the 'far' radio button selected. When the image comes up, select 'Operation','Change Position' and then locate the viewing position to where you want it.

The results of the effort can be seen in the images in figures (3) and (4) to the right. It might have looked a little better with the hydro layer added, but I could not get this one to render properly. Even so, the results are satisfactory. The great advantage of this method is that it opens up a large new library of DLG overlay material. Thanks to Paul for showing us a way to combine the best features of two fine programs to make superior DEM overlays. You can see an example of Paul's work and other excellent DEM overlays at the 3DEM Gallery.