Problems Using SDTS 7.5' DEMs?
Why can't my software read the USGS SDTS DEMs hosted by GeoCommunity?
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
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: