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There are two main CRS definition authorities that are relevant for the AI domain: Oil and Gas Producers (OGP), formerly known as the European Petroleum Survey Group (EPSG) and the OGC. The two sets of CRS definitions have many common points. For example, the
OGC:CRS84 is a variant of EPSG:4326 (differing only in its coordinate order: longitude/latitude). The EPSG CRS database is available at http://www.epsg.org - European Petroleum Survey Group Geodesy Parameters [EPSG CRS]. Recommendations for the use of CRS references in GML data sets are provided in the OGC Recommendation Paper “URNs of definitions in ogc namespace”recommendation paper 07-092r3
. The CRS of an AIXM geometry is identified by a URN (e.g. urn:ogc:def:crs:EPSG::4326) and defined in its srsName attribute.
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For all geometries (Surface, Curve, ElevatedPoint, etc.) a CRS shall be specified. The CRS is either defined directly on the geometry element using the srsName attribute or is derived from the larger context the geometry is part of.
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