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11.G.iii. Which Computer Are You Using?

Because the DEM data are stored in a 16-bit binary format, users must be aware of how the bytes are addressed on their computers. The DEM data are provided in IBM PC-compatible byte order, which stores the least significant byte first ("little endian"). Systems such as Digital Equipment VAXes and Alpha workstations, as well as computers using most Intel-compatible central processing units (CPUs) use this byte-ordering scheme.

For Apple Macintosh computers and most popular UNIX workstations, the typical byte order is most significant byte first ("big endian"). Some UNIX GIS applications provide an option to swap the bytes when importing the data (e.g. ERDAS/IMAGINE) or the information can be specified in header files (e.g. Arc/Info and ArcView). If the application does not provide this option (e.g. GRASS), users with systems that use big-endian byte ordering have to "swap bytes" of the DEM data. This is easily done on most UNIX workstations, using the "dd" command. Check how this command works on your UNIX workstation, probably by running the command "man dd." Typically, the command is executed as follows:

dd if=inputfilename of=outputfilename conv=swab


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