Sometimes the area you are interested in mapping may not line up exactly with a single USGS quadrangle. In this case You may need to edge match and stitch two or more DEMs to makeone composite DEM. This was the case for a terrain map of a hike I made a couple of years ago with my daughters from Elkmont to Clingman's Dome in the GreatSmoky Mountains National Park. Clingman's dome falls in two states and four USGS 1:24,000 quads (Gatlinburg, TN; Mt. LeConte, NC; Clingman's Dome, NC and Siler's Bald, NC). I could have cropped down the appropriate 1:250,000 quad but this would not have yielded the degree of terrain detail that I wanted.

One way to effect the merge is to simply combine the .tgas derived from the DEMs in Paintshop or Photoshop and then render in POV-Ray. I started by converting each DEM to a grayscale TGA image using MicroDEM. Using PaintshopPro, I rotated the images, added them as layers to a base image, and aligned the edges. These USGS quads align fairly well at the edges but some rotating and trimming was required to get a good match with no overlap or gaps.

Because of the way that elevation space is mapped to grayscale space by the conversion program, the grayscale coloration was not scaled consistently among the four base images. This resulted in some edge lines in the mosaic image. I used the processing features in Paintshop Pro to adjust the coloration to minimize this problem. Some faint edge lines are visible in the renderedimage but they are not really noticeable. In order to solve this problem correctly, the mapping function in the conversion program needs to be modified. I have written an extension to the earlier DEM2TGA2 called DEM2TGA4 to incorporate this feature.

When I was done adjusting, I merged the layers and saved the image as a TGA file. I created a green texture in Paintshop Pro to mimic the dense forest covering thisarea of the Smoky Mountains and saved it as a TGA as well. I overlayed this texture image using DEMDRAPE2 .The result is shown to the right. (Note: the four source DEMS are not used directly to create the composite. Instead the TGAs are merged. They are shown for illustrative purposes only.) You may combine any number of "tiles" by this technique into your mosaic and build quite large DEMS. Of course, storage and processing requirements increase proportionately