Choices in How We Make Representations - A Primer of GIS: Fundamental Geographic and Cartographic Concepts - page 45

complicated representations between a geographic representation and the
phenomena on which it is based. Because of scale's significance for data col-
lection and processing, indicating the scale for geographic information pro-
vides crude but effective shorthand for understanding the accuracy of geo-
graphic information. Large-scale maps cover small areas with significant
detail and accuracy. Medium-scale maps cover larger areas with less detail
and accuracy. Small-scale maps cover large areas with the least detail and
accuracy.
The United States Geological Survey describes the positional accuracy
of their topographic products in terms of the U.S. National Map Accuracy
Standard (1947) that states 90% of the points tested should fall within a fixed
distance (0.02 inch or 0.5 mm) of their correct position. The points tested
are only the well-defined points, which leaves open the possibility that less
well-defined points are far less accurate.
In terms of positional accuracy, it would be too easy to simply say that
an accuracy standard means you know what you're getting. Remember that
the actual location of these points could be anywhere within the areas indi-
cated. In this sense, while the positional accuracy standard is a quantitative
measure, its interpretation is often very qualitative unless exhaustive mea-
sure is made.
Scale also impacts attribute accuracy: fewer detailed measurements and
observations can be made and represented at smaller scales. Smaller scales
(larger areas) must generalize and combine phenomena into cartographic
features that remain visible at the output or data analysis scale. Determining
attribute accuracy is more complicated and generally involves comparisons
of selected points on one map or in one geographic information data set
with another or field checks at the actual locations. These tests are very
important as they often indicate important differences between pieces of
geographic information that are supposedly of the same phenomena, but in
actuality are collected and/or analyzed using very different methods.

Quality and Choices

As discussed in Chapter 1, a common way to think about the reliability of a
geographic and cartographic representation is in terms of quality. The high-
way map is “good” if we can use it to find our way easily. Simply said, good
quality geographic information or maps are useful for the purpose we create

TABLE 2.1. U.S. National Map Accuracy Standard Calculations for Three
Common U.S. Topographic Mapping Scales (0.02 inch or 0.5 mm)

Scale Accuracy in ground units

1:24,000 40 feet

1:63,360 105.6 feet

1:100,000 (Metric) 416.6667 meters