A complex format for the transfer of spatial data between disparate computer systems. The driver reads data sets encoded using the Raster Profile and Extension (RPE). The driver is not a fully compliant reader of RPE data sets, but it does successfully read the DEMs distributed by the USGS.

The driver reads raster data sets of any data type supported by RPE. All descriptive information is read as well. Multiple rasters can be read from a single transfer, and every raster can have multiple bands.

Successive reading of SDTS data is not very efficient, so you may want to copy the data to a more efficient format before processing. These successive reads will actually only happen when the machine that is running DSpatial is low on memory. Since SDTS transfers are sized to accommodate common computer capabilities "as defined by current technology" this will only be a concern on older or stripped-down computers.

Certain exotic option in the RPE are not supported (tiling of data, compression, external files, etc).

The driver does not support writing of new or modified data sets.

Coordinate system information is included in the exchange. This information is processed and an accurate coordinate system is generated, with the exception of the State Plane Coordinate System (SPCS). All coordinate systems are assumed to use the meter as the linear unit of measure.

The SDTS driver is actually composed of a family of units. SDTS transfer files are encoded in the ISO8211 format. The DSpISO8211.pas unit parses any file (not just SDTS files) encoded in that format. The DSpSDTS.pas unit parses the SDTS modules, as defined in the SDTS standard, using the output from the ISO8211 parser. The DSpSDTS_RPE.pas implements the RPE profile, using the parsed modules. The DSpSDTSDriver.pas unit, finally, implements the DSpatial driver.

An SDTS transfer actually consists of about 15-20 files, each containing the data for one or more modules. Most files are in ASCII format, but files containing large amounts of data are typically binary. All files belonging to one transfer have the same first four characters in their file name, followed by four characters indicating the module in the file, and having a .DDF extension. For instance, the xxxxCATD.DDF file contains the catalog that identifies all files that belong to the transfer xxxx.

USGS DEM transfers are named something like 1660190.DEM.SDTS.TAR.GZ. As that name implies, this is a gzipped, tarred set of files. The file names start off with four digits, which indicate the map sheet that the DEM belongs to. Unfortunately, these numbers (and thus the entire file name) are used for adjacent DEMs, so each transfer should be unzipped into its own directory (as is prescribed in the standard).