A site calibration is a series of mathematical computations that transform WGS84 coordinates into local grid coordinates. A GNSS receiver measures WGS84 latitude, longitude, and ellipsoidal height coordinates to determine its location, however, surveyors typically use a local grid coordinate system. To benefit from the increased accuracy of modern RTK GNSS surveying, the WGS84 coordinates need to be translated into local grid coordinates. This can be accomplished by working in a pre-defined coordinate system with known control, or by performing a GNSS site calibration. Essentially, a GNSS site calibration establishes the relationship between WGS84 latitude, longitude, and ellipsoidal height, with the needed local northing, easting, and elevation. A site calibration typically consists of a horizontal and a vertical adjustment.

A site calibration is a series of mathematical computations that transform WGS84 coordinates into local grid coordinates.

The horizontal adjustment is a least squares best fit to make the GNSS derived grid coordinates fit, as closely as possible, to the local control grid coordinates. The parameters resulting from this are a rotation, translation, and a scale of the GNSS coordinates. The vertical adjustment can be done with or without a geoid model. If you do not have a geoid model, a simple vertical shift as a single point can be applied, or an inclined plane can be computed using the orthometric heights of the local control.

If you use a geoid model, an inclined correction plane is created based on the residuals between the geoid defined by the orthometric heights of the local control and the applied geoid model.