Problems Using SDTS 7.5' DEMs?

Prior to the change in distribution of SDTS DEMS from a USGS website to the commercial GeoCommunity site, the USGS reprocessed all of its DEMs to fix a positional error problem introduced by the previous mass conversion (circa 1997.) Also at the same time the USGS made two other changes in the data: 1) The SDTS Raster Profile, finalized in 1999, had a minor change in its profile identifier (which is not reflected in the new DEMS); 2) The DEM product changed to decimal units for the elevation values so most DEMs made after 1999 use a data type of floating point (i.e. 810.25) rather than the previous integer type (i.e. 850). These changes collectively affected all of the USGS SDTS DEMs. Some GIS software may be unable to read the new SDTS DEMS because of either or both of these changes. Users should contact their software vendor to find the latest information on software upgrades or patches. Users who rely on free or public domain software might check the Web discussion lists for the latest solutions.

If the problem is only the profile identifier, then a workaround is available from the USGS. Please note that use of the workaround does not make the data compliant to the FGDC standard and it should not be considered a fix. The issue of non-compliance is with the profile identifier name alone and does not affect the integrity of the data. FGDC-compliant data will replace the existing data and will be available from the GeoCommunity by October 1, 2001.

More information about the SDTS Raster Profile Change (Aside: A profile is a subset of a larger standard where options are narrowed and certain field content is restricted. It is much easier for software vendors to develop applications based on profiles rather than the entire base standard with numerous options.)

The first mass conversion of the USGS DEM to the SDTS Raster Profile occurred while the SDTS Raster Profile was a near final draft. This is not an unusual practice. The USGS had a large part in developing SDTS and the Raster Profile. Encoding our data is the strongest endorsement we could give.

There was a minor change between the draft and final versions of the Raster Profile (see Table 1). The draft version requires the string (a) "SDTS RASTER PROFILE" to identify the profile used to encode the data. This string (a) occurs in all DEMs that were downloaded from the USGS web site, and consequently identifies a dataset with the geopositional error. The ratified Raster Profile now requires the string (c) "SRPE: SDTS RASTER PROFILE and EXTENSIONS". USGS modified their encoding software to use this new identifier, and to support decimeters and to correctly handle geopositioning. Several sample datasets were produced. These samples were made available to provide software vendors advanced notice of all of the changes. Unfortunately, these samples contained a variant of the profile identifier. We used the profile identifier of (b) "SRPE: RASTER PROFILE and EXTENSIONS", inadvertently omitting the term "SDTS". All SDTS DEMS currently being served from the GeoCommunity web site are encoded with string (b).

TABLE 1-- Variance in Profile Identifiers
a) Draft Identifer
SDTS RASTER PROFILE

b) New USGS DEMs
SRPE: RASTER PROFILE and EXTENSIONS

c) Final Profile Identifier
SRPE: SDTS RASTER PROFILE and EXTENSIONS

The USGS holds standards in the highest regard and we intend to ensure the integrity of our data. FGDC-compliant data will replace the existing data and will be available from the GeoCommunity by October 1, 2001. If you believe there may be a problem with an indiviaual data set or if you need additional information about SDTS, please contact the SDTS Task force at this email address: sdts@usgs.gov.

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