Geoborder for Airspace Border

In the Aeronautical Information (AI) Domain, airspace boundaries may be based on national borders or on other geographical features, such as shorelines, rivers, etc.

An example of such an Airspace border is provided below:

“UBP3

400300N 0455323E - 400300N 0465600E - 392545N 0472148E -

then along the state border with Islamic Republic of Iran up to 385222N 0463250E -

then along the state border with Armenia up to 400300N - 0455323E”

A particularity of this situation is that official definitions of the airspace, as provided in the Aeronautical Information Publication (AIP) or in NOTAM messages, do not include the actual geometry of the referenced geographical border. It is left for the end users to derive the actual geometry of the airspace by using a source of geographical border data.

The UML model of AIXM shows a “dependency” association between Surface and GeoBorder to cater for such situations.

The GeoBorder class is used to encode a physical or political border. In general, it will be the border between two countries or states but could also be a coastline, the description of the bank of an important river, or any other geographical shape which can be named and used to describe the border of an airspace. If two countries/states have more than one common border, each one will be an occurrence of this entity.

The encoding of GeoBorder references can be done in two ways:

• either using the annotation property of the aixm:Curve class, for applications where a simple text remark is sufficient;
• or using the xlink:href attribute of the gml.curveMember element, for applications where a true reference needs to be preserved.

Using aixm:Curve annotations

In this case, the aixm:Curve is used as content of a gml:curveMember, which allows including an aixm:annotation. This encoding has the advantage that the geometry is self-contained (the relevant series of latitude/longitude pairs from the referenced GeoBorder are directly copied in a gml:GeodesicString/gml:posList element).

This method should be used when the information (that this part of the airspace border actually comes from a GeoBorder) is intended “for human consumption”, such as for directly displaying the Airspace on a screen or printing on paper. An example is provided below:

TBD

Note that the first point of the curveMember that contains the portion extracted from the GeoBorder (39.42916667 47.36333334) is equal with the last point of the previous curveMember. This satisfies the requirement stated in the GML Standard (ISO 19136, 10.5.11.1 - Ring, RingType, curveMember) that “In the context of a ring, the curves describe the boundary of the surface. The sequence of curves shall be contiguous and connected in a cycle.” In order to match this requirement when encoding existing Airspace data an additional gml:curveMember might be necessary. This is required in order to connect the last specified point of the Airspace boundary before the GeoBorder with the first vertex of the GeoBorder. It is typically a very short segment which does not alter the geometry. However, its presence is important in order to have a contiguous Surface border.

Using xlink:href

When necessary to preserve as a true reference the (that a part of the airspace border actually comes from a GeoBorder), then a gml:curveMember with a xlink:href attribute can be used. In this case, there shall be no child aixm:Curve or gml:Curve element. The GML standard requires a local reference, using a gml:id value. For compatibility reasons with previous AIXM versions and to satisfy the operational needs of the AI domain, it is also allowed to use a remote reference. The two solutions are detailed here.

For the detailed AIXM/GML encoding, see the document Guidance and Profile of GML for use with Aviation Data.

Coding Examples

The figure below shows the encoding of an airspace referencing a geoborder as part of one of its geometry components.

More examples TBD in the scope of the DONLON AIXM AIP data set.