In GML, the geodetic datum is specified by reference to a Coordinate Reference System (CRS). A CRS relates a coordinate system to the Earth by a datum.

A geodetic datum consists of an ellipsoid model and a prime meridian. The intersection of the equator and prime meridian (at which longitude is defined to be 0°) is the origin of the CRS.

A geodetic CRS (e.g. EPSG 4326) relates a (lat/long) ellipsoidal coordinate system to the Earth.

Usually the information about the geodetic reference datum is specified once for the whole AIP and not provided for each individual position.

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: 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.

The examples below show the encoding of the same Airport Reference Point (ARP) with different CRS applied. In the first example (OGC:1.3:CRS84), the longitude value comes first, then the latitude. In the second example (EPSG:4326), the latitude value comes first and then the longitude.

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.

Unless overruled by the presence of a local srsName for convenience in constructing feature and feature collection instances, the value of the srsName attribute on the gml:Envelope shall be inherited by all directly expressed geometries in all properties of the feature or members of the collection. As indicated in the Figure below, the gml:Envelope is a child element of the gml:boundedBy property of the feature. If a geometry uses the same coordinate reference system as given on the gml:boundedBy property of its parent feature that geometry does not require a srsName attribute,

Inheritance of the coordinate reference system continues to any depth of nesting. However, if overruled by a local srsName declaration, then the new coordinate reference system is inherited by all its children in turn.

Notwithstanding this rule, all the geometries used in a feature or feature collection may carry srsName attributes. This is in order to indicate a local reference system, even if they are the same as the parent. A geometry without a srsName derives its CRS from its closest ancestor that has a srsName. Due to the way that AIXM is modelled, this can only be one of the following:

aixm:Surface
aixm:Curve
aixm:Point

If no such ancestor is found, that geometry without srsName derives its CRS from the srsName of the gml:Envelope in the boundedBy element of the feature or feature collection in which the geometry is contained. Geometries for which no CRS can be derived are invalid.

Specifying the srsDimension attribute is not required (see XML encoding examples of an ARP above). This is because it is implicit from the CRS identified by the srsName. Specifying the srsDimension could lead to discrepancies, such as using srsName =”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, 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.

When encoding aeronautical data that complies with the WGS-84 ICAO Standard, the following Coordinate Reference System (CRS) shall be used in AIXM 5.1:

EPSG:4326 - for data conforming to the usual aviation practice (latitude first, longitude second, angles/bearings measured from the North with positive values clockwise);
OGC:CRS84 - for data conforming to the more “mathematical” practice (longitude as first axis, latitude as second axis, angles measured from the East with positive values counter-clockwise).

When encoding aeronautical data that does not comply with the WGS-84 ICAO Standard an appropriate CRS shall be used.

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