What GCPs (ground control points) are used for?

Ground Control points (GCPs) are not obligatory for good 3D reconstruction. But they can be used for extremely accurate Georeferencing and scaling, meaning that the measurements will be even more accurate and the placement of the reconstructed terrain is flawless on an absolute scale.

In case of perfect conditions consumer grade GPS device (such as drone) can provide horizontal accuracy up to 5 meters and vertical up to 10. With professional survey grade ground control points, accuracy of a few centimeters can be achieved. As an alternative Real Time Kinematic (RTK) or Post Process Kinematic (PPK) systems can be used which utilize a base station for GPS signal correction in real time or in post-processing. Drones and cameras equipped with RTK or PPK compatible devices are an order of magnitude more expensive but provide comparable accuracy to survey grade GCP's while eliminating the need to physically measure GCP's.