Why is the number of ground control points twice as much as the theoretical need?

In order to improve the accuracy and reliability of photogrammetry, the number of ground control points needs to be twice the theoretical number.

In photogrammetry, ground control points are the basis for determining the transformation relationship between image coordinate system and ground coordinate system. Photogrammetry involves large image scale and relatively low image resolution, so it is necessary to improve the accuracy and reliability of photogrammetry through ground control points. Moreover, in actual operation, due to the interference of various noises and errors, it is difficult for a single ground control point to meet the requirements of high precision. Therefore, it is necessary to set up multiple ground control points for simultaneous calculation to improve accuracy and reliability. Generally speaking, the number of ground control points is 1 times more than that required by theory, which can ensure the accuracy and reliability of the calculation results.

In addition to increasing the number, attention should also be paid to the layout of ground control points, which should be distributed as evenly as possible in the survey area and cover the whole area. At the same time, we should also consider the spatial position and elevation distribution of control points, as well as the accuracy requirements of the measurement and marking steps of ground control points.