Transformations - Spatial Mathematics: Theory and Practice through Mapping - page 72

been collected at 1:24,000 scale. Owing to modern, fine screen resolution, the
features may look like they are in their “true” locations. However, a buffer
zone of accuracy, if it were available, would show rings around each feature,
such as buildings and water wells. From those rings, the data user would be
astounded to see the many locations possible for these features. Therefore,
knowing the spatial resolution of the data is key to making wise decisions
with it. One should not make decisions at a finer scale than that at which the
data were collected.
3.4.2 Vector data resolution: Considerations
Vector data resolution has to do with the positional accuracy of the features
represented, and the attributes of those features, the “G” and the “I” compo-
nents of a GIS, respectively. Many digital data sets in use today were originally
produced in paper map form and scanned or otherwise converted into digital
formats, and thus carry with them the accuracy standards of their original
maps. They are no more accurate in the digital world than they were in the
analog world. Standards for paper and digital mapping indicate the tolerance
for error; they do not imply that maps or the digital data produced from maps
are perfect.
For example, as we have seen earlier and will see again later, one of the first
things done with mapped data is to choose what projection they should be
cast in. Projections by their very nature are distortions of reality, in area, dis-
tance, direction, and/or shape. In terms of horizontal accuracy, consider the
US National Map Accuracy Standards: For maps on publication scales larger
than 1:20,000, not more than 10 percent of the points tested shall be in error
by more than 1/30 inch, measured on the publication scale; for maps on pub-
lication scales of 1:20,000 or smaller, 1/50 inch. These limits of accuracy shall
apply to positions of well-defined points only. In terms of vertical accuracy,
the National Map Accuracy Standards state “Vertical accuracy, as applied to
contour maps on all publication scales, shall be such that not more than 10
percent of the elevations tested shall be in error by more than one-half the
contour interval.” (Read more about the National Map Accuracy Standards
on http://egsc.usgs.gov/isb/pubs/factsheets/fs17199.html. ) Again, this idea
implies that some features will in fact be off by more than these amounts.
Thus, when we think about map accuracy, error must be managed; it cannot
be eliminated. In terms of the attributes in a vector data set, resolution has to
do with such things as the number of attributes a data set contains, and how
many different categories of each attribute it contains (such as the Anderson
land use classification scheme that contains several resolutions of complete-
ness (Anderson, 1976)). The data user must determine whether the accuracy
of location and attributes can serve the project requirements; in other words,
whether any given data set is “it for use.” It is the data producer's responsibil-
ity to document the accuracy and completeness for any data set in the set of
metadata provided to the user.