Significant Point for Airspace Border

Introduction

Positions of navaids or designated points may be used in the aeronautical information domain to define the shape of an airspace. They can be used as arc centers or even as boundary points. In the case of NOTAM messages, the latitude/longitude coordinates may be followed by geographical references, such as in the example below.

E) AIR DISPLAY WILL TAKE PLACE WI LATERAL LIMITS: 443838N 0200818E (NDB OBR) - 444508N

0201455E (VILLAGE JAKOVO) - 443445N 0202447E - 443838N 0200818E (NDB OBR).

F) GND G) 3000FT AMSL)

This is not always a reference to a significant point, it can be simply an annotation of a position (e.g. “Village Jakovo”). However, the encoding solution being quite similar, this case also is discussed here.

The UML model of AIXM shows a “dependency” association between Surface and SignificantPoint in order to cater for such situations:

The reason for using a “dependency” association and not a standard “object to feature association” is that this occurs deep inside the GML encoding of the Surface. Using a “dependency” also indicates that the actual encoding can be done differently, based on the intended use of the data.

The encoding of point references and annotations can be done using gml:pointProperty elements, which can appear as a descendant of gml:Curve, for example in the construction of a gml:GeodesicString.

More details about the use of pointProperty are provided in a dedicated section of the general coding rules for geometry.

Coding Examples

The figure below shows a significant point (i.e. NDB "DESI") used to define an ArcByCentrePoint for a geometry component of an airspace (see the page Arc by Centre Point for detailed coding instructions and issues related to such kind of geometry).

The radius, the start angle and the end angle of the arc is defined based on the location of the NDB.

For the angle values the spatial reference system used (srsName) has to be taken into account. In this example a SRS is applied (OGC.1.3:CRS84) that requires the longitude value first and then the latitude value second.

More coding examples can be found in the AIP Data Set - Specimen (DONLON).