Parallels

In the AI domain, if an Airspace border has two consecutive points at the same geographical latitude, it is assumed that the line between the two points is “along the parallel”. This shall be encoded in AIXM/GML using “linear” GML elements in combination with a geodetic CRS, such as EPSG:4326. The linear interpolation in a 2D geodetic CRS between two points that have the same latitude corresponds to a parallel on the Earth’s surface. This is shown in the figure below:

For example, a LineStringSegment with two coordinates (i.e. a line segment with begin and end point) may be used with a srsName that references a Mercator projection (e.g. EPSG:3395), which is a well supported conformal projection. Note that the LineStringSegment element implies that linear interpolation must be used and srsName = ”urn:ogc:def:crs: EPSG::3395” implies that the interpolation is done in the Mercator projection plane. Hence this geometry gets realized as a rhumbline on the WGS84 ellipsoid Earth model. Other conformal stereographic and conical projections can also be used to represent rhumblines on the earth ellipsoid model of WGS84 (this may be useful for regions of interest near the poles).

In conclusion:

• in the classical aeronautical information case of two consecutive points having the same latitude, a gml:LineStringSegment with "default" CRS EPSG:4326 shall be used for encoding. This is the case for most Airspace data published by States.
• in the particular case where one wants to express a rhumbline whereas the two consecutive latitudes are different, a gml:LineStringSegment with a "Mercator" CRS like EPSG:3395 may be used. However, this is a theoretical discussion since no real world aeronautical data like is known to require the use of arbitrary rhumblines.