INTERNAL: Uncompression Considerations In Opening USGS SDTS DEM Files

In certain situations, attempting to open a USGS SDTS DEM may produce the following error message:

An error occurred while converting the SDTS file to an image. Error: Ath SX is out of range

The problem lies in how the file(s) have been compressed / uncompressed.

Many USGS SDTS DEM files are distributed with multiple layers of compression, e.g. some combination of tar and zip. How these files are uncompressed can determine if a file is intact or corrupted. In particular, the *rsdf.ddf file may be affected. Using tar and zip from some common Windows utilities may produce an invalid *rsdf.ddf file. The dem will open in the USGS program dlgv32 as it does not use this file, but it will not open in ENVI. Uncompressing these files on a Linux or UNIX machine or through a Linux emulator for a PC (such as cygwin, an open source Linux emulation for PC) should fix the behavior and produce DEM files from USGS SDTS DEMs that are valid and useable in ENVI. This problem can be seen in comparing the results of the two uncompressions and noticing the difference in file size from the two approaches for the *rsdf.ddf file.