Control point
In photogrammetry and remote sensing , control points (also: reference points , English Ground Control Point (GCP)) are used to determine the elements of the orientation of a measurement image. These are points in the terrain whose position is known in a corresponding (terrain) coordinate system and which can be clearly recognized in an aerial or satellite image .
There are three types of control points:
- Full control points for which the spatial coordinates X, Y and Z are known,
- Location control points at which the location coordinates X and Y are known and
- Height control points at which the height coordinate Z is known.
Furthermore, two types of points come into question as control points: If a particularly high accuracy of the control points is necessary or if there are no natural landmarks in the area of the recording, points with known coordinates must be signaled prior to the flight. Normally, however, natural points in the terrain are chosen; this can be, for example, house corners or crossroads.
Obtaining control points
Maps are mainly used to obtain location control points, but the derivation of height control points using contour lines is also conceivable. It must be ensured that the referencemap has a scale thatcorresponds to the aerial or satellite image, that the map has a sufficient number of usable control points and that no generalization has taken placeat the control points.
GPS enables the relatively quick acquisition of control point coordinates in the field. However, the prerequisite is that there is a good connection to the satellites. The use of differential GPS (DGPS) leads to very precise coordinates, as required in many applications.
Aerotriangulation is mainly used to determine the absolute orientation of stereo models in an image group. The control points are obtained from the images themselves if some control points are already known.
Geodetic measurement of control points is time-consuming and costly, which is why this method is only used in exceptional cases. However, it gives the most accurate results.