Based on accuracy statements made for USGS topographic quadrangle
maps. Targeted to National Standard for Spatial Data Accuracy (NSSDA).
Target coverage coordinate precision is double precision to accept up to
15 digits.NHD coordinates are created in decimal degrees with 7 digits
to the right of the decimal place (e.g. .1234567). This level of
precision in decimal degrees equates to between approximately .02 and 1
meter in projected coordinate units on the ground (depending on the
latitude of the coordinate). Additional offsets to positions may have
been introduced where there are many features to improve the legibility
of map symbols. This occurs because much of the source data was derived
from a cartographic source (DLG or CFF). As positional accuracy is
improved through edits this cartographic constraint will be removed. In
addition, the digitizing of maps is estimated to contain a horizontal
positional error of less than or equal to 0.003-inch standard error (at
map scale) in the two component directions relative to the source maps.
Visual comparison between the map graphic (including digital scans of
the graphic) and plots or digital displays of points, lines, and areas
is used to assess the positional accuracy of digital data. Linear
features of the same type along the adjoining edges of data sets are
aligned if they are within a 0.02-inch tolerance (at map scale). At
1:100,000 scale, 0.02 inch is approximately 167 feet (50.8 meters), and
at 1:24,000 scale, 40 feet (approximately 12.2 meters).
The National Map Accuracy Standards for 7.5 minute 1:24,000
quadrangles applies to this coverage, which state that a feature will be
within approximately 40 feet of its true position.