Although the terms used to describe the problem vary widely-"accuracy", "uncertainty", "error",
"quality"
are all used, and with substantial overlap of meaning-the core issue is really quite simple.
Traditionally, maps have been viewed as comfortable, scholarly, pleasant, civilized things, to be
displayed on walls just as we display books on shelves. But over the past 30 years we have
brought the contents of maps into the precise, brutish world of digital computers, where they are
treated as measurements of the world, or collections of facts with fixed, well defined meaning.
When the subjective and somewhat vague content of a map is digitized, analyzed, and the results of
analysis returned to the user, the authority of the digital machine replaces that of the map author
or the collector of the data. Suddenly, contours that were drawn to convey an impression of the
form of the Earth's surface become the basis for precise measurements of elevation, slope, or
aspect, to as many digits of precision as the machine can produce.