Ground control points (GCP)

A ground control point segment contains up to 256 ground control points. Ground control points are normally used to associate projection coordinates with locations on a raw (uncorrected) image; however, they can theoretically be used to relate locations in any two georeferencing systems.

Each ground control point defines a location in each of two georeferencing systems: the first georeferencing system (normally raw image coordinates), and the second georeferencing system (normally some projection system). The definitions of the georeferencing systems are kept as 16 character map units strings, with "PIXEL " indicating raw image coordinates.

Since projection parameters are not stored, only projections that are fully defined by the map units string, can be used (e.g., UTM, Long/Lat, or METRE). Complex projections such as Transverse Mercator cannot be used. It is hoped that this limitation will be relaxed in the future.

Each of the control points can also have an elevation associated with the location in each image. An elevation unit string is kept for each system; it may have the values of "METRE" (the default) or "FEET".

Each ground control point has the following values associated with it:

• Id: A unique numeric identifier for the control point. If it is negative, it is interpreted as indicating that the point is a check point, and should not contribute to the transformation model.
• System 1 X: The X coordinate in the first georeferencing system. This is normally a pixel location in the image.
• System 1 Y: The Y coordinate in the first georeferencing system. This is normally a line location in the image.
• System 1 Elevation: The elevation of the location in the first georeferencing system. This is normally zero, and ignored by applications.
• System 2 X: The X coordinate in the second georeferencing system. This is normally a location in projection coordinates.
• System 2 Y: The Y coordinate in the second georeferencing system. This is normally a location in projection coordinates.
• System 2 Elevation: The elevation in the second georeferencing system. This should be zero if it is not used.

There is currently no way of storing more than 256 GCPs, nor of associating symbolic names with the control points.