Geographic Transformation

A geographic transformation is a mathematical operation that converts the coordinates of a point in one geographic coordinate system to the coordinates of the same point in another geographic coordinate system.

Because geographic coordinate systems contain datums that are based on spheroids, a geographic transformation also changes the underlying spheroid. There are several methods, which have different levels of accuracy and ranges, for transforming between datums. The accuracy of a particular transformation can range from centimeters to meters depending on the method and the quality and number of control points available to define the transformation parameters.

A common transformation in North American data is between NAD83 and WGS84, or between the GCS North American 1927 and GCS WGS 1984 geographic coordinate systems as shown below.

A geographic transformation always converts geographic (latitude–longitude) coordinates. Some methods convert the geographic coordinates to geocentric (X,Y,Z) coordinates, transform the X,Y,Z coordinates, and convert the new values back to geographic coordinates.

See Also

• Understanding Geotransformations; ESRI Developer Network
• Geodetic Datum Overview - Peter H. Dana, Univ. of Colorado Boulder

• Esri blog: About geographic transformations and how to choose the right one