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&emdash;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.