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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:
longitudelatitude and is defined in the ISO 19128 Geographic information — Web Map Server standard. 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” [OGC 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 or derived from the larger context that the geometry is part of.
When applied to the encoding of a surface in AIXM, this will give the following GML element:
aixm:Surface gml:id
= S01
srsName =
urn:ogc:def:crs:EPSG::4326 Specifying the srsDimension attribute is not required. This is because it is implicit in the
srsName. Specifying the srsDimension could lead to discrepancies, such as using srsName=”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 EPSG::4979.
6.3. WGS-84
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”.
The Coordinate Reference System CRS reference is critical for the correct encoding and processing of AIXMGML geometries. This is because a CRS not only indicates the geodetic
datum and ellipsoid for which point coordinates are expressed but also the order of the coordinate axes in which coordinate values are provided, e.g. latitude before longitude – which
is an important convention for the aviation domain.
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 5.1GML data sets that contain arcs of circle defined by start angleend angle measured from the North. This will be explained in section
Measuring angles in GML of this document.
Therefore, the EPSG:4326 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, anglesbearings 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. .
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When encoding aeronautical data that does not comply with the WGS-84 ICAO Standard an appropriate CRS shall be used. A list of CRS that are likely to be used in aeronautical data sets
is provided in Annex A of this document. The use of other CRS falls outside the scope of this document.
Geographic coordinateslatitude and longitude shall be expressed in decimal degrees, not in degrees minutes seconds.
6.4. Use of global srsName
