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Usually the information about the geodetic reference datum is specified once for the whole AIP and not provided for each individual position.
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| title | Note |
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The Coordinate Reference System (CRS) is critical for the correct encoding and processing of the geographical data contained in AIXM/GML files. The CRS indicates not only the geodetic reference datum, but also the order of the coordinate axes (latitude/longitude or longitude/latitude) and has important implications for the convention used for measuring angles (from the North, clockwise, for example).
The CRS of an AIXM geometry is identified by a URN (e.g. urn:ogc:def:crs:EPSG::4326) using an srsName attribute.
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. Recommendations for the use of CRS references in GML data sets are provided in the OGC 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.
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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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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Specifying the srsDimension attribute is not required (see XML encoding examples of an ARP above). This is because it is implicit in from the CRS identified by the srsName. Specifying the srsDimension could lead to discrepancies, such as using srsName =”EPSG”urn:ogc:def:crs:EPSG::4326” and srsDimension =”3”. One could think that this is a good way to describe 3D WGS84 coordinates. However, this assumption is wrong and an appropriate 3D srsName should be used in that case, such as "urn:ogc:def:crs:EPSG::4979".
According to ICAO Annex 15
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