discussion #7917
Updated by Andreas Kohlbecker over 5 years ago
A spatial reference system (SRS) or coordinate reference system (CRS) is a coordinate-based local, regional or global system used to locate geographical entities. Some of the default terms provided by the `eu.etaxonomy.cdm.model.location.ReferenceSystem` class, do not conform to a reference system: * WGS84: Is the World Geodetic System 84 also known as WGS 1984, EPSG:4326, this one is perfectly ok * GoogleEarth: makes no sense if names like this. GoogleEarth is the name of an application. This term should be renamed to one of Web Mercator, Google Web Mercator, **Spherical Mercator**, and has the code is EPSG:900913 * Gazetteer: is a geographical dictionary or directory used in conjunction with a map or atlas, it is not a reference system and must be removed * Map: makes no sense at all as reference system, remove it! **Background information on geodetic reference systems:** A geodetic datum or geodetic system is a coordinate system, and a set of reference points, used to locate places on the Earth (or similar objects). Reference systems are composed of a couple of constructs like an ellipsoid, geoid, anchor coordinates, etc. **ellipsoid** ** ellipsoid** In geodetic coordinates, the Earth's surface is approximated by an ellipsoid. An Earth ellipsoid is a mathematical figure approximating the Earth's form, used as a reference frame for computations in geodesy, astronomy, and the geosciences. Various different ellipsoids have been used as approximations. **geoid** The geoid is the shape that the ocean surface would take under the influence of the gravity and rotation of Earth alone, if other influences such as winds and tides were absent. This surface is extended through the continents (such as with very narrow hypothetical canals). For us it is important to note that a geodetic reference systems is an ideal approximation to the shape and form of the earth. Any coordinate (longitude / latitude) defines a point in this geometric structure. **A coordinate in a geodetic system can not be compared to the coordinate of another system without having the knowledge on the reference systems.** Links: * https://en.wikipedia.org/wiki/Geodetic_datum * https://de.wikipedia.org/wiki/Geod%C3%A4tisches_Datum * https://en.wikipedia.org/wiki/Reference_ellipsoid * https://en.wikipedia.org/wiki/Geographic_coordinate_conversion