Hi Gordon, Ideally you want as many as you can and should have them well spread. There is no limit on how many you can have, however the more you have the better the accuracy. Please ensure that you spread them out well, if you put them all in a line it will not fix the model it will just rotate on an axis created from the gcp’s. A good way would be to have them at either side of the highway running along the stretch.
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