estimated to be 98.5 percent. One or more of the following methods were used to test attribute
accuracy: - manual comparison of the source with hardcopy plots. - symbolized display of the
digital line graph on an interactive computer graphic system. - Selected attributes that could not be
visually verified on plots or on screen were interactively queried and verified on screen. In
addition, softwarevalidated feature types and characteristics against a master set of types and
characteristics, checked that combinations of types and characteristics were valid, and that types
and characteristics were valid for the delineation of the feature. Feature types, characteristics, and
other attributes conform to the Standards for National Hydrography Dataset (USGS, 1999) as of the
date they were loaded into the database. All names on reaches were validated against a March
1999 extract from the Geographic Names Information System. The entry and identifier for the names
match those in the Geographic Names Information System. The association of each name to
reaches has not been methodically checked, and so a name may be applied to the wrong reaches.
Anecdotal reviews indicate that 80 percent or more of the named reaches have the correct name.
Reaches were delineated with a batch procedure and were checked extensively during the visual
pass steps of processing. Based on automated quality assurance/quality control checks performed
at various intervals during the processing, approximately 99 percent of the reaches are delineated
according to standards.
intersect only at nodes, and all nodes anchor the ends of lines. Lines do not overshoot or
undershoot other lines where they are supposed to meet. There are no duplicate lines. Lines bound
areas and lines identify the areas to the left and right of the lines. Gaps and overlaps among areas
do not exist. All areas close.
were compiled to meet National Map Accuracy Standards. For horizontal accuracy, this standard is
met if at least 90 percent of points tested are within 0.02 inch (at map scale) of their true position.
Additional offsets to positions may have been introduced where there are many features to improve
the legibility of map symbols. 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). To align the features, the
midpoint between the end of the corresponding features is computed, and the ends of features are
moved to this point. Features outside the tolerance are not moved; instead, a feature of type
connector was added to join the features.