we have been doing some test with two P4 and photogrammetry. By taking a soccer field we get a elevation difference of 1.5 to 2.1 m from one end to the other. Consistently, we get an elevation increase from the beginning of the flight path to its end. Also, multiple flights, one after the other, give different elevation readings. This means that taking ground control points, either with a total station or a RTK gps is an absolute need in order to produce accurate maps. Does anyone know how reading is taken for the exif data? Is it a combination between GPS and barometric? GPS alone should not produce such behaviour. Maybe the surroundings of the barometric probe inside the drone are progressively eating up. Strange and annoying, as in many cases we would not need the exact ASL elevation but just a correct terrain shape and we could avoid wasting time taking ground reference points.