- a "...Limit" attribute, which includes the value and its unit of measurement (as an "uom" attribute) with the list of values: UomDistanceVerticalType;
- a "...LimitReference" attribute, which indicates what reference system is used for the vertical limit value. It uses the following list of values: CodeVerticalReferenceType;
The following consistency rules between the uom value and the reference value shall be observed when encoding the data:
Coded values (GND, UNL, etc.)
In addition to numerical values, the "...Limit" attributes can use four coded values:
- GND - meaning "from surface";
- UNL - meaning "unlimited";
- FLOOR - meaning "from the bottom of the airspace/route, whatever that one is";
- CEILING - meaning "to the top of the airspace/route, whatever that one is"
- The values FLOOR and CEILING may be used only in AirspaceLayer, in relation with AirspaceActivation, AirspaceLayerClass, RouteAvailability).
As the unit of measurement attribute is mandatory in AIXM for all “...Limit” attributes, the following values are recommended when encoding the data:
in Digital NOTAM can be expressed as numerical values or as coded values (such as GND - meaning "from surface"). They shall be coded as indicated in the general AIXM coding rules for Vertical Limits.