Merging DEM Files

Sometimes the area of interest on a DEM falls on the border between to files. For this reason, the DEM Reader can merge any two DEM files. After two files have been merged, the area on the edge of file will be in the center.

The program can combine two files that are adjacent in the east-west direction or in the north-south direction. For locations that fall at the corners of four DEMs, you can combine west-east and then south-north.

The merge capability has been tested with one degree and 7.5 DEM files. It may be able to process other scales, but these have not been tested. The program is designed to process the original DEM format and but not the new SDTS format files. If you are using SDTS format data, you must convert it to the original format first.

Merging Files. The file merging utility window has two pages to it. One for merging west-east files. One for merging south-north files. The first step merging files is to select the proper page.

Selecting Files. Each page has two edit boxes for the west and east files or the south and north files. You can type the filename into each box, or you can use the browse button to search for and select the proper files.

Pan Factor. Once you have selected the files to merge, you can set a Pan Factor. The Pan Factor controls how much of each file will appear in the new file. The default Pan Factor is 50%. A Pan Factor of 50% means the half of each file will appear in the new file. Smaller Pan Factors mean more of the west or south files will appear in the new file. Large Pan Factors mean more of the east or north files will appear in the new file.

After you have selected files and a Pan Factor, pressing the "Merge" button. The program will prompt you for an output files and then combine the two files.

Errors And Problems. Several error conditions can occur when you try to merge two files. Here is detailed description of the errors:

1. Non-Adjacent Files. If the files are not adjacent, the program will tell you how far apart the edges are and ask you really want to merge the files.

2. Anomalies. Anomalies may exist at the border between two files. There are two types of anomalies. The first kind is caused by missing points or gaps at the border of the files. These are due to inconsistencies in the way the DEMs are created by the USGS. The second type of anomaly is the result of elevation offset between the two files. For example, the elevations of one file could be consistently 30 feet higher than the other file. This would result in a hump at the boundary of the file.