Curve (Line) and Surface

Certain features in the AI domain have polygonal shapes (such as Airspace) or linear shapes (such as a power line Vertical Structure). They are typically published (for example, in Aeronautical Information Publications – AIP) as a series of latitude/longitude positions, such as in the following example:

EAP 25 (The Castle)

521108.00N 0051230.00E;

521222.00N 0051715.00E;

521121.00N 0051756.00E;

521009.00N 0051756.00E;

(then along the parallel to) 521009.00N 0051311.00E;

to point of origin.

Usually, the interpolation method used for the curve between the consecutive points it is not indicated in the AI source documents , but it is generally assumed that:

• If two consecutive points have the same latitude value, then the line connecting the two points is a parallel on the surface of the Earth; this may be explicitly stated using words such as “along the parallel to” (for the detailed encoding see page Parallels);
• Otherwise, it is considered a “straight line on the map” (for the detailed encoding see page Straight Line).

In addition, the source map used when the airspace was designed is typically unknown.

Arcs of circle are also used in the definition of airspace borders, such as in the following

examples:

EHR 4A (VLIEHORS) TSA

531012.59N 0044621.14E; along clockwise arc (radius 8 NM, centre 531500.00N 0045700.00E) to 530701.98N 0045602.41E;

531100.00N 0045124.00E; to point of origin.

EHR 4B (VLIEHORS)

530943.06N 0050658.79E; 530240.00N 0051500.00E; 525809.00N 0050622.00E; 530701.98N 0045602.41E; along anticlockwise

arc (radius 8 NM, centre 531500.00N 0045700.00E) to point of origin.

Arcs may also be used in the definition of approach/departure trajectories. However, specific “path and terminator” codes are used to encode such arcs and the use of GML in this case is limited to providing a curve for printing the procedure on a map. GML is not used to encode the real flight trajectory of an aircraft, as stored in the Flight Management System (FMS).

Curve

In AIXM 5 lines are encoded using the Curve or ElevatedCurve element, which are extensions of the gml:Curve. 

For the ElevatedCurve class, also elevation may be specified (e.g. the elevation of a fence).

They are typically published as a series of latitude/longitude positions.

The pairs of lat/long coordinates can be encoded as either a sequence of gml:pos or, more compact, using a gml:posList element.

The same separator (space) is used both between the latitude and longitude values (coordinate separator) and also between the latitude/longitude groups (tuple separator).

The figure below shows the encoding of a Taxiway Guidance Line.

The figure below gives an example of the encoding of a fence that is a Vertical Structure.

Surface

In AIXM 5 lines are encoded using the Surface or ElevatedSurface element, which are extensions of the gml:Surface. 

For the ElevatedSurface class, also elevation may be specified (e.g. the elevation of a building that is an obstacle).

Surfaces are encoded in GML using gml:PolygonPatch elements.

The pairs of lat/long coordinates can be encoded as either a sequence of gml:pos or, more compact, using a gml:posList element.

They are closed shape, i.e. the first latitude/longitude pair has to be equal with the last one.

As stated in section 10.5.11.1 of the GML Standard:

Every gml:curveMember references or contains one curve, i.e. any element which is substitutable for gml:AbstractCurve. 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 the special case that there is only one curve member in a gml:Ring, this means that the curve member itself needs to form a cycle, thus the need for the last position to be equal with the first one.

The figure below shows the encoding of an Elevated Surface for a Taxiway Element.

The figure below shows the encoding of an Elevated Surface for an Apron Element.