GCPs

Ground Control Points, or GCPs, are points whose world coordinates are known by survey and whose image coordinates can be accurately digitized from an image.

They can be any identifiable feature, for example the intersection of white pavement markings or a manhole cover. Or they can be targets that have been added to the scene for just this purpose, for example the checkerboard patterns in Figure 1.

Figure 1. Example checkerboard GCP on the FRF pier. The target is about 1 m square in order to be visible from high-flying UAVs. The cement block keeps the target from blowing away in the wind and should not cover the target center.

The world location of each GCP, [x,y,z], must be found by survey in local coordinates, at a target point that can easily be identifiable in the image. For the checkerboard, that location would typically intersection of the border lines. For a manhole cover it would likely be the geometric center. The corresponding image location of each GCP, [U,V], must also be determined, often by zooming in and clicking in the image. It should be realized that pixel resolution is often around 10 cm, so both surveys and image location picking need to be to an accuracy that is commensurate with pixel accuracy or required accuracy of subsequent analysis methods. Often it is desirable to identify GCP locations to better than an accuracy of one pixel.

Accurate image coordinates can often be found using a center of mass (COM) calculation. If the target is visually identifiable within a local region by being either brighter or darker than the surroundings, the GCP pixels can be found by thresholding and the center of mass found (the mean of the U and V locations of all the thresholded pixels). This method is usually accurate to a fraction of a single pixel, often called sub-pixel accuracy. This is the method used in the UAV toolbox in the routine called findCOMRefObj.m (in the first 30 lines of code).