Geographic Information Systems: An Introduction, 3rd Edition

11.2 Measuring Accuracy and Precision

Accuracy versus precision

Although commonly considered to be identical, accuracy and precision express different aspects of measurements and should be carefully distinguished. Accuracy expresses how closely a measurement represents the quantity measured, or closeness of agreement between a test result and the accepted reference value. Precision expresses the repeatability of the measurement. For the measurements normally of interest in GIS, precision is limited only by the instruments and methods used. In physical measurements, precision is customarily indicated by the number of significant figures carried in a quantity expressed. For instance, a figure of 101.23 cm indicates that the distance is measured with a precision of a fraction of a millimeter. The last decimal place may be uncertain. It is thus quite possible to measure with high precision and produce a result that is inaccurate. One can envision a situation where a measurement is made with high precision in inches, but a wrong result is given in centimeters.

Accuracy should be stated in terms of an interval in which a true value is assumed to lie. For instance, 101.21 ± 0.04 cm indicates that the true value is assumed to lie between 101.17 and 101.25 cm. However, some textbook authors may use a notation with estimated value ± standard deviation. Care should be taken to avoid misunderstandings.

The terms accuracy and precision are particularly relevant in relation to geometry and quantitative ...