GEOG 862
GPS and GNSS for Geospatial Professionals

Local and Network Accuracy

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diagram showing local and network accuracy, see text below
Local and Network Accuracy
Source: GPS for Land Surveyors

Local and Network Accuracy

The use of the phrase network accuracy is used to define its difference from local accuracy. Network accuracy, here, concerns the uncertainty of a position relative to a datum. Local accuracy is not about a position relative to a datum, but it represents the uncertainty of a position relative to other positions nearby. In other words, local accuracy would be useful in knowing the accuracy of a line between the two positions at each end. Network accuracy would not be about the accuracy of the positions at each end of the line relative to each other, but, rather, relative to the whole datum.

Local Accuracy is also known as relative accuracy and network accuracy is also known as absolute accuracy. The network and local accuracy values provide very different pictures. The local category represents the accuracy of a point with respect to adjacent points. The network category represents the accuracy of a point with respect to the reference system.

Local horizontal and vertical accuracies represent the averaged uncertainty in points relative to adjacent points to which they are directly connected. Local horizontal coordinate accuracy is computed using an average error radius between the point in question and other adjacent points. Height accuracy is computed using an average of the linear vertical error between the point in question and other adjacent points.

Within a well-defined geographical area, local accuracy may be the most immediate concern. However, those tasked with constructing a control network that embraces a wide geographical scope will most often need to know the position's relationship to the realization of the datum on which they are working. A point with good local accuracy may not have good network accuracy, and vice versa.

Typically, network horizontal and vertical accuracies require that a point's accuracy be specified with respect to an appropriate national geodetic datum. In the United States, as a practical matter, this most often means that the work is tied to at least one of the Continuously Operating Reference Stations (CORS) which represent the most accessible realization of the National Spatial Reference System in the nation.