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”;
- Otherwise, it is considered a “straight line on the map”.
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).