GPS results can be evaluated by analyzing repeat base line differences, network loop misclosures and residuals from a minimum constraint least squares adjustment. It was noted in the second article that if GPS users follow the NGS guidelines, they will reduce and/or eliminate errors in ellipsoid height and, at a minimum, detect problems or errors in data. It was also mentioned that the basic concepts are very simple, but they all need to be followed exactly as prescribed. For example, “the observing scheme for all stations requires that all adjacent stations (base lines) be observed at least twice on two different days and at two different times of the day.”
GPS can provide “absolute” and relative positioning information much easier, faster and more precisely than some classical techniques. However, the wrong station can still be occupied, the height of the antenna can be measured wrong or incorrectly entered during the base line reduction processing phase, the receiver can malfunction, an abnormal atmospheric condition can cause large errors in the height component, or some “unknown Gremlin” can cause an error source.