Hi Gordon, Not easy to answer in fact. I didn't fly enough with my mavic to talk about GCPs distribution. In my point of view, the important element is the GSD I want to obtain at the end of the mission. If the terrain is easy (flat, not to much vegetation), I fly at the same height to have a constant GSD on the whole project. If the height and the GSD are defined, then the flight plan can be defined too, so the GCPs plan too. I never fly at different heights in mountains with my Ebee. It can't follow the terrain. Finding a landing zone is quiet difficult. The risk to crash is too high. I often station on the top, and if I have several stations, I try to find spaces at nearly the same altitude. Not good for the GSD in the bottom part in the valley... Flying at different heights with a multicopter in mountains is possible. So I don't think the distribution of the GCPs is a real problem in mountains, the GSD and the overlap between flights do... Plan flights following the terrain will be the best situation. If not, I would plan flights by ensuring the GSD, like stairs, so that there wouldn't be to much difference between two flights, and ensuring the overlap between flights. " Considering that the flight height and the GSD have a linear relationship, for the same project, captured with the same camera, the highest flight height at which images are taken should not exceed two times the lowest flight height: GSD1 ≤ 2 * GSD2 (Sw * H1 * 100) / (Fr * Iw) ≤ 2 * (Sw * H2 * 100) / (Fr * Iw) H1 ≤ 2 * H2 Where: GSD = Ground Sampling Distance [cm/pixel]. Sw = sensor width [mm]. H = flight height [m]. Fr = real focal length [mm]. Iw = image width [pixel]." Defining the GSD gives the height. The height, the direction of the capture (1 or 2 directions) and the battery life give the surface of the flight. The surfaces of the flight and the project give the number of flights. Then, knowing the number of flights, the average accuracy of my Mavic, and the difficulty of the terrain (fields, mountains, forests, deserts, buildings etc) I can choose the number of GCPs to implant. A visit in site can help. It can be 1GCP per flight, or 1 GCP on the overlap of 2 flights, or 4 flights, or less... It depends of the geometric accuracy I want to obtain. Because of the average accuracy, I think I would implant 1GCP per flight. It means flights between 30 to 80m high, in 2 directions, a GSD between 1 to 2.5cm/px, a flight surface between 1 to 2.5Ha (in only 1 direction, the surface doubles). That's my opinion about my multicopter put in situation of my own experience. Not an advice and not a rule. Each site is different and flight planning must be well prepared. Implanting GCPs is quiet long to do. I always try to do it in only one day. But sometimes it can be longer. It depends of the size of the area to survey, and the time I can spend on site. It's a mission in the mission. So I wouldn't be too careful on the number of GCPs, especially on difficult terrains (mountains, forests, cities). It's better a little more than not enough. Hope that makes sense and helps...
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