REQUIREMENTS ON THE HEIGHT BENCHMARKS IN THE THIRD PRECISE LEVELLING OF SWEDEN

The development in the field of surveying is very intensive today. The instrumentation as total stations, levelling instruments and GPS-equipment is constantly improved in order to be easier to use and to achieve better accuracy of the measurements. Calculation programmes are also improved for the same reasons. However, most measurements are related to some kind of reference points in the terrain that should carry the coordinates or the heights calculated from the measurements, so what happens if the benchmarks does not meet the standard of the measurements is that the good measurements performed are loosing a bit of its value. The efforts made to achieve high-class measurements are more or less wasted. Or can we really afford to make those accurate measurements just for the pleasure to see that nice low RMS in the result files from the calculation? Not only the stability and accessibility of the benchmark is important, but also to be sure that the point used in the field is the right one. To guarantee that, it is necessary to have a reliable identification and documentation of all the points. The quality of a measurement can easily be judged by calculation, and we can accept the measurement or remeasure, but how can the quality of a benchmark be judged? Unfortunately this can usually not be done until the point is used the next time, perhaps after several years, and then it is too late. This paper mainly describes the requirements on the benchmarks in the third precise levelling of Sweden and what is done in order to meet those requirements. The actions taken here can however be applied to most kinds of networks.