# GeoTransformation

A class that defines datum transformation parameters.

Property |
Type |
Description |
---|---|---|

WKID |
int |
A well-known id of a datum transformation. For a list of valid WKID codes for transformations, see the discussion Finding a Well-Known ID. |

WKIDSpecified |
bool |
If true, an id is used to define the transformation. |

WKT |
string |
The well-known text string used to define explicit parameters of a transformation. |

## Remarks

All the elements in a map layer have a specific geographic location and extent that enables them to be located on or near the earth's surface. The ability to accurately describe geographic locations is critical in both mapping and GIS. Coordinate systems, sometimes known as map projections, provide a common basis for communication about a particular place or area on the earth's surface. There are two types of coordinate systems: geographic and projected.

A geographic coordinate system uses a three-dimensional spherical surface to define locations on the earth. It includes an angular unit of measure, a prime meridian, and a datum (based on a spheroid). In a geographic coordinate system a point is referenced by its longitude and latitude values.

A projected coordinate system is defined on a flat, two-dimensional surface. Unlike a geographic coordinate system, a projected coordinate system has constant lengths, angles, and areas across the two dimensions. A projected coordinate system is always based on a geographic coordinate system that is based on a sphere or spheroid.

Coordinate systems enable you to integrate datasets within maps as well as to perform various integrated analytical operations such as overlaying data layers from disparate sources and coordinate systems. This class provides access to methods that help ensure the proper geographic alignment of MapServer image output. Though MapServer does utilize a predetermined set of geographic transformations, the default transformation may not apply in all cases. To ensure spatial accuracy you may need to specify a GeoTransformation.

Use this object when the image output from a map service is to
be displayed in a coordinate system where the underlying geographic
coordinate system is different than the underlying geographic
coordinate system of the MapServerInfo.*DefaultMapDescription*.

For example, a given map service may be using a spatial reference system based on GCS_WGS_1984. A client wants to display the map service in a different spatial reference, such as GCS_European_1950. If no GeoTransformation is applied the resulting map image will use the spatial reference of the DefaultMapDescription, GCS_WGS_1984, and will display incorrectly in GCS_European_1950. Features may not "line up" correctly. In order to correctly display these results a GeoTransformation is needed. Applying the correct GeoTransformation ensures the spatial correctness of the result.

A GeoTransformation is not needed if different projected coordinate systems share the same underlying geographic coordinate system. For best results it is optimal for the data (layers within the map service) and the map service to use the same coordinate system.