SDTS DEM Positional Error (Source: USGS)
One positioning error is isolated to the spatial domain module of the SDTS
file. This module contains the UTM corner coordinates of the file's bounding
rectangle. Because of coordinate rounding errors, these bounding corner
coordinates may be off as much as one meter in x and/or y from their proper
positions. This rounding error, however, does not affect the positions of
the internal DEM post coordinates. Because the bounding rectangle was
calculated independently from the internal coordinate positions, it is
possible that some internal post coordinates may actually lie as much as one
meter outside the footprint of the bounding rectangle. Programs that do not
attempt to register the DEM tiles using coordinates of the spatial domain
module will not be affected by this coordinate rounding error.

The other positioning error affects the coordinates of the internal DEM
posts for SDTS 10-meter and 30-meter DEMs. This problem results from the use
of an incorrect origin for the SDTS UTM grid calculated for each 7.5-minute
tile. Rather than placing grid posts at 10- or 30-meter increments from a
consistent origin within the respective UTM zone, the 10- or 30-meter
step-over in northing and easting was calculated from the minimum easting
and maximum northing (northwest corner) of the target DEM cell. The error
for each SDTS tile will vary depending on the orientation of the UTM grid to
the 7.5-minute latitude and longitude of the original DEM corners. The x or
y error between any grid post's correct horizontal position and calculated
position will not exceed the spatial resolution of that particular DEM. This
error will be consistent for all elevations within a 7.5-minute tile but may
vary across tiles. When this error varies across tiles, column or row
drop-out may be evident along the common edge of those tiles.

There is no simple offset or translation that can be applied to the existing
SDTS DEMs to correct for these conversion errors. The USGS is presently
considering the proper action to be taken regarding this problem. These
problems have been fixed in the SDTS conversion software, therefore, any
SDTS DEMs created after January 1, 2001 will contain correct positioning.


My understanding is that this was a problem when using SDTS DEM files with
Bryce.  Bryce may very well not account for the projection but I thought
this could be what you were seeing.

Post a reply to this message