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There are two main CRS definition authorities that are relevant for the AI domain: International Association of Oil & Gas Producers (IOGP), 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. Recommendations for the use of CRS references in GML data sets are provided in the OGC recommendation paper 07-092r3.
For GML encoding, the latitude/longitude data needs to be expressed in numerical (degrees with decimals) format, not in degrees minutes seconds. No sign or a plus sign means North or East respectively. A minus sign means South or West respectively. If this is not the case with the originator data, a data format conversion should take place. This conversion should be done with a sufficient number of decimal values in order to preserve the original precision of the data. In order to avoid the introduction of small imprecision due to such format conversions, data originators shall be asked to send the data in the raw numerical format (degrees with decimals) that is typically used for survey and geodetic calculations.
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According to ICAO Annex 15, all “published aeronautical geographical coordinates (indicating latitude and longitude) shall be expressed in terms of the WGS-84 geodetic reference datum”.
Due to the way that angle directions are traditionally measured in the AI domain (North corresponds to 0°, East to 90°, etc.), the use of the OGC:CRS 84 is not straight-forward for AIXM 5.1/GML data sets that contain arcs of circle defined by start angle/end angle measured from the North. Therefore, the EPSG:4326 Coordinate Reference System (CRS) is the typical choice for AIXM 5.1 data sets that use the WGS-84 reference datum. However, this does not exclude the use of other CRS when appropriate.
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When encoding aeronautical data that does not comply with the WGS-84 ICAO Standard an appropriate CRS shall be used.
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