Precision

In the geographic sciences, as in mathematics, precision refers to the number of significant digits, exactness, or detail to which a value has been reliably measured. That is, it describes the finest unit of measurement used to express a measured value. For example, if a record is reported to the nearest minute, the precision is 1/3,600th of a degree; if a decimal degree is reported to two decimal places, the precision is 0.01 of a degree. In statistics, on the other hand, precision refers to the ability of a measurement to be consistently reproduced or the level of agreement among a series of measurements, values, or results (i.e., its repeatability or reproducibility).

Although a location or attribute may be measured very precisely, it does not mean that it is necessarily accurate. Accuracy is the closeness of a measurement to the actual (true) value. Thus, a measurement or value may be precise but not accurate, accurate but not precise, both accurate and precise, or neither accurate nor precise.

Obtaining or recording precise data can often be costly and time-consuming, and in many cases, high precision may not be needed or suitable. For example, it is unnecessary to record a road junction to the centimeter when all you want to do is find where you need to turn when driving.

Sometimes, it is difficult to determine the precision of a given geographic representation. For example, many boundaries or lines on a map or in a geographic database may have low precision due to the method by which they were derived (e.g., from contour lines) or because they may vary over time (the course of a river or the coastline). However, when drawn as a line on a map, they may appear to be very precise.