Ground Control Points (GCPs)
What is a Ground Control Point
The Ground Control Points (GCPs) are specific pixels in an image for which the output map coordinates are known. GCPs consist of two X,Y pairs of coordinates:
- Source coordinates—usually data file coordinates in the image being rectified
- Reference coordinates—the coordinates of the map or reference image to which the source image is being registered
GCPs are identifiable features located on the Earth’s surface that have known ground
coordinates in X, Y, and Z. A full GCP has X,Y, and Z (elevation) coordinates associated with it. Horizontal control only specifies the X,Y, while vertical control only specifies the Z.
The following features on the Earth’s surface are commonly used as GCPs:
- Intersection of roads
- Utility infrastructure
- Intersection of agricultural plots of land
- Survey benchmarks
In the type of mapping project, GCPs can be collected from the following sources:
- Ground GPS
- Total station survey
- Theodolite survey
- Planimetric and topographic maps
- Digital orthorectified images
- Digital elevation model
Requirements for minimum GCPs for an accurate mapping project vary with respect to the size of the project. With respect to establishing a relationship between image space and ground space, the theoretical minimum number of GCPs is two GCPs having X, Y, and Z coordinates and one GCP having a Z coordinate associated with it.
In order to compute a unique solution, at least seven known parameters must be available. In using the two X, Y, Z GCPs and one vertical (Z) GCP, the relationship can be defined. However, to increase the accuracy of a mapping project, using more GCPs is highly recommended.
Ground Control Point Markers
Collect GCPs in QGIS
This is a Georeferencing process to Marks GCP.
Collect GCPs in ArcGIS
GCPs in ERDAS IMAGINE
The GCP set is stored in the image file along with the raster layers. If a GCP set exists for the top layer that is displayed in the Viewer, then those GCPs can be displayed when the Multipoint Geometric Correction tool.
In the CellArray of GCP data that displays in the Multipoint Geometric Correction tool or GCP Tool. One column shows the point ID of each GCP. The point ID is a name given to GCPs in separate files that represent the same geographic location.
Collect GCPs in GPS
The GPS device helps you to collect accurate Ground Control Points, then it’s use in GIS or Remote Sensing Software.
Example of GPS Survey to collect GCPs or Waypoint.