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- 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 TBD Straight Line).
In addition, the source map used when the airspace was designed is typically unknown.
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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.
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The figure below gives an example of the encoding of a fence that is a Vertical Structure.
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
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.
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