Accuracy depends on many factors among which, the flight height, type of camera used, the accuracy of the ground control points…Accuracy can be as good as 5 mm. Although, this accuracy is obtained thanks to the latest (and expensive) technology. It also requires much more time for the setup, the flight and the process. The absolute accuracy can only be as good as the accuracy with which the ground control points are collected. It thus depends on the tool that is used to measure the gcp’s: GNSS receiver or total station.
You should count on an average accuracy of 2 cm horizontal and 3 cm vertical.
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The relative accuracy however can be as low as a mm. Indeed, ground control points are used to calibrate the model but not to position it in a reference frame. Aetos drones achieved an a survey of the cooling tower of Tihange Power Station.