The gcp full form in gis Ground Control Point. Ground Control Point is a point of geospatial interest on the Earth’s surface that provides positional data to allow for calculation and measuring of distance and other geographic information.
Ground Control Points (GCPs) are points in space usually defined by five or more GPS measurements that provide robust coverage to the surrounding area while still maintaining their relative integrity. GCPs go hand in hand with GPS surveys; they describe points on the Earth’s surface, such as an airport runway centerline, street centerline, monument corners or building corners. Usually one ground control point is enough to define a survey area but for high accuracy requirements it may be necessary to define at least two control points. The complexity of computation used will depend on how accurately it is required to locate the surveyed points.
The focus of GCPs is on accurate positioning rather than intelligent surveying which can be used to supplement existing surveying techniques. For example, if a traditional surveyor’s transit was not available for surveying but you did have access to a GPS receiver, an approximated location (x, y) of the target could be calculated by setting up a control point at the observed location.
GCPs are points that have known coordinates, typically defined by GPS receivers. GCPs are used for conducting surveys in order to determine the position of objects or features in space. A Ground Control Point can be any object found in an aerial image, virtual or real-world. Any object with a known coordinate is considered a potential GCP when it can be registered with another geolocated object through triangulation using satellite images. The registration process uses the multilateration method and requires at least five ground control points and satellites overhead to achieve this accuracy level. This system does not require laser scanning equipment and therefore ideal for very large areas.
The most accurate method of surveying is using Real-Time Kinematic (RTK) methods. RTK requires a base station and rover to be present in the area of interest, as well as suitable satellite coverage. The base station receives corrections from one or more sources and calculates them into Cartesian coordinates which are then broadcast to the rovers for survey results. A time delay must be allowed between corrections received and broadcasts sent so that each radios can calculate and transmit corrections independently, this is known as “differential correction”. A GCP solution based on ground control points with RTK uses its own base station with an external reference receiver such as those found in Real-Time Locating (RTLS).
If coordinates are known of two GCPs anywhere on the Earth, measurements to points in between can be made by assuming an ellipsoidal model of the earth.
Ground control points are used for the following data set types:
There are numerous applications for GCPs including : topographical surveys, transportation planning, architecture and construction management, accident scene mapping, access analysis & planning studies, seismic hazard studies, municipal planning studies and as a reference frame for orthorectification techniques such as those used by satellite sensors . It also helps to provide accurate geospatial details about ground level locations without having any information from a traditional survey. This provides significant time and cost savings compared to manual surveys because it is more efficient than its alternatives.
The three main types of GCPs are permanent, temporary and historical. Permanent GCPs are used in imagery that is collected for a long time period where the location has not changed significantly. Temporary GCPs are created by placing equipment at an object, acquiring images from an overhead sensor, digitizing features on the acquired images to form new control points or the creation of “Ground Truth” control points. The objective of temporary GCPs is to capture existing features such as buildings and fences which might change over time so they can be referenced back to a common point for all images using interferometry techniques. Historical GCPs were once created manually but now sky-scanning robotics reduces the cost and timeline dramatically under Survey-to-Model programs. Historical GCPs can be used to create a digital elevation model by using light detection and ranging (LIDAR) which is then used to produce contours and digitize features into image space.