Geographic Information Systems: An Introduction, 3rd Edition

11.1 Selection Criteria

The use of analog maps leads us to assume that all objects on a map are of similar quality. This is not the case with GIS, where data of differing origin and accuracy may well be mixed. In addition, the introduction of GIS into mapping has produced a completely new (and perhaps less professional) user group and producer group. In such a situation, an understanding of the concept of quality will be very important, and the marking of data according to its degree of quality a necessity.

The applications of digital data are indeed many and varied, but digital data systems are not inherently more accurate than the analog systems they supplant. In general, digital systems are capable of processing data more precisely than analog systems, but their overall accuracy still depends on the accuracy of their source data, which in most cases remain analog.

To see why this is so, let us consider time, perhaps the most accurately measured quantity in everyday life. In the 1950s, extremely accurate ships' chronometers and other timepieces became available, at a high price; ordinary wristwatches and clocks were accurate to only a few minutes a day. In most applications, clock accuracy was a question of how much a user was willing to spend for a finely made mechanical timepiece. National standards of time were kept by using large electronic clocks that derived their signals from the oscillations of quartz crystals housed in temperature-controlled ovens. These clocks were ...