Accuracy

The accuracy of a single measurement can be defined as:

An accurate measurement has a mean close to the true value. Measurement errors are generally described in terms of accuracy. In the case of spatial data, accuracy may relate not only to the determination of coordinates (positional error) but also to the measurement of quantitative attribute data. This can include positional accuracy, temporal accuracy, or attribute accuracy.

In the case of surveying and mapping, the “truth” is usually taken to be a value obtained from a survey of higher accuracy, for example by comparing photogrammetric measurements with the coordinates and heights of a number of independent check points determined by field survey. Although it is useful for assessing the quality of definite objects, such as cadastral boundaries, this definition clearly has practical difficulties in the case of natural resource mapping where the “truth” itself is uncertain, or boundaries of phenomena become fuzzy.

Prior to the availability of GPS, resource surveyors working in remote areas sometimes had to be content with ensuring an acceptable degree of relative accuracy among the relative and absolute accuracy measured positions of points within the surveyed area. If location and elevation are fixed with reference to a network of control points that are assumed to be free of error, then the absolute accuracy of the survey can be determined.