Q: We recently ran a GPS traverse between two monuments of different orders about 21 miles apart. We set traverse monuments about two to three miles apart. When we got to the second monument, we missed the published state plane coordinates by about 0.5 ft. What gives?
Manufacturers of GPS receivers state accuracies for the receivers in their instrument material. These accuracies will vary depending on the manufacturer, but for this discussion, let's use a value of Â±(5 mm + 1 ppm). Hopefully, you were doing static observations and not RTK; if you were using RTK, the value may have to be more like Â±(1 cm + 2ppm). In general, RTK is not recommended as a method of extending control. If the control stations you worked with were 21 miles apart, and we assume you did not set up your traverse stations in a straight line, let's assume there were eight lines at three miles apiece. The random error (the generally used standard for the manufacturer accuracy statements) for each line will then be about Â±1 cm. For the eight lines, the expected random error from the GPS receivers only will be about 2.8 cm (1 cm âˆš8) or 0.09 ft.