The terms “accuracy” and “precision” do not refer to the same idea. Both are important to GIS. A classic example of the difference follows:
Weather forecaster A indicates that it will be between 40 and 50 degrees tomorrow at 4 p.m. The actual reading turns out to be 43. Thus, the forecast was accurate, but not very precise. Forecaster A provided a true statement but without much detail. Forecaster B states that it will be 52.47 degrees at 4 p.m. tomorrow. The temperature turns out to be 43 degrees. Forecaster B was very precise, but not accurate.
The terms accuracy and precision can also be applied to a set of readings or measurements. Again, a classic example: Darts are thrown at targets. The target of person “R” looks like Figure 6-4.
Her dart throwing was both accurate and precise. The darts are where they are intended to be (accurate), and very close together (precise).
Person S’s target looks like Figure 6-5.
The darts are tightly clustered (they hit a spot almost precisely) but lack accuracy, as they missed the intended spot. (Perhaps some sort of systematic error is present—such as a breeze blowing up and to the left.)
Person T’s target is as shown in Figure 6-6 ...