Overview

The term accuracy is used to express the degree of closeness of a measurement, or the obtained solution, to the true value. The term precision, however, is used to describe the degree of closeness of repeated measurements of the same quantity to each other. In the absence of systematic errors, accuracy and precision would be equivalent [1]. For this reason, the two terms are used indiscriminately in many practical purposes. Accuracy can be measured by a statistical quantity called the standard deviation, assuming that the GPS measurements contain no systematic errors or blunders. The lower the standard deviation is, the higher the accuracy.

For the one-dimensional case, for example, when measuring the length of a line between two points, the accuracy is expressed by the so-called root mean square (rms). The rms is associated with a probability level of 68.3 percent. For example, the accuracy of the static GPS surveying could be expressed as 5 mm + 1 ppm (rms). This means that there is a 68.3 percent chance (or probability) that we get an error of less than or equal to 5 mm + 1 mm for every kilometer. In other words, if we measure a 10-km baseline, then there is a 68.3 percent chance that we get an error of less than or equal to 15 mm in the measured line.

Horizontal component (e.g., easting and northing) accuracy, a two-dimensional case, is expressed by either circular error probable (CEP) or twice distance rms (2drms). CEP...