Surveying using GPS
Photogrammetry, (the technique for making maps from aerial photographs), and tying satellite imagery to its true ground position require accurate positions for ground features which can be clearly identified on the images or photos. We use a technique called Differential GPS Surveying to measure these 'ground control points'. This method involves having one GPS receiver on an already accurately known survey point, the 'base-station' collecting GPS data simultaneously with other 'roving' receivers measuring new, previously unsurveyed points. We usually collect GPS data for one hour at each point. During post-processing using special software the GPS positions at the base station are compared with the known position and GPS errors in latitude, longitude and height calculated. When these are applied as correction factors to the data from the roving receivers, along with information derived from the changing positions of the GPS satellites during the survey, results with a few centimetres accuracy can be generated for the new surveyed points.