The USGS states the horizontal accuracy of their DEM's is 130m
with a circular error at 90percent probability.
The following is a discussion of accuracy from the USGS Data User
Guide for the 90m DEMs:
The accuracy of a DEM is dependent upon its source and the spatial
resolution, that is grid spacing, of the data profiles. One factor
influencing DEM accuracy is source data scale and resolution. A dependency
exists between the scale of the source materials and the level of grid
refinement possible. The source resolution is also a factor in determining
the level of content that may be extracted during digitization. For example,
1:250,000-scale topographic maps are the primary source of 1-degree DEM's.
Horizontal accuracy of DEM data is dependent upon the horizontal spacing of
the elevation matrix. Within a standard DEM, most terrain features are
generalized by being reduced to grid nodes spaced at regular intersections
in the horizontal plane. This generalization reduces the ability to recover
positions of specific features less than the internal spacing during testing
and results in a de facto filtering or smoothing of the surface during
gridding.