Experiences with GPS assisted aerial triangulation in The Netherlands

摘要

Determination of the relative positions of photo projection centers using the Global Positioning System (GPS) has become an important tool for aerial triangulation. This statement holds true even for a country, such as The Netherlands, where a lot of ground control is available. The use of GPS only marginally increases the costs of the photo flight, but creates the potential for a large reduction in ground control. In The Netherlands there is a network of five permanent GPS reference stations available. Therefore, there is no need to set up a dedicated reference receiver in the project area. However, the relative precision of the projection centers is only slightly better than 10cm standard deviation. With ground precision requirements at the centimeter level, ground control cannot be reduced to the theoretical minimum of only a few points. The paper discusses a practical test with GPS assisted aerial triangulation, investigating the possibilities for reducing the amount of ground control. The results of three bundle adjustment computations are analysed. For the first computation a standard amount of ground control was available. In the second computation the amount of ground control was reduced by almost 50%. In the third computation all ground control was used, but the GPS observations were not. The average spatial shifts of the tie points of the second and third computations, relative to the first, were in the same order of magnitude. However, the exclusion of the GPS observations affected mainly the vertical direction homogeneously over the block, while the reduction in ground control locally degraded the precision of the tie points in all directions. It was concluded that a major reduction in ground control degrades the precision too much to be acceptable, and that GPS is essential for meeting the precision requirements in height.