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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 fileís 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 postís 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.

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