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:
Note that the
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| The current GML 3.2.1 does not allow general “rhumbline” (constant angle with the meridians) interpolations to be specified directly (e.g. using a RhumbLine element or a “rhumbline” interpolation). However, linear interpolations in certain conformal projections do correspond to rhumblines on the ellipsoid Earth model. |
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).
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