Spatial reference system

A particular SRS specification (for example, "Universal Transverse Mercator WGS 84 Zone 16N") comprises a choice of Earth ellipsoid, horizontal datum, map projection (except in the geographic coordinate system), origin point, and unit of measure.

Thousands of coordinate systems have been specified for use around the world or in specific regions and for various purposes, necessitating transformations between different SRS.

There are also internal systems for measuring location within the context of an object, such as the rows and columns of pixels in a raster image, Linear referencing measurements along linear features (e.g., highway mileposts), and systems for specifying location within moving objects such as ships.

Virtually all major spatial vendors have created their own SRID implementation or refer to those of an authority, such as the EPSG Geodetic Parameter Dataset.

SRIDs are typically associated with a well-known text (WKT) string definition of the coordinate system (SRTEXT, above).

Earth Centered, Earth Fixed coordinates
Earth centered, Earth fixed coordinates in relation to latitude and longitude.
Layout of a UTM coordinate system.