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Numerical values

Vertical limits are typically encoded in AIXM as a pair of two attributes:

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:

Reference	Reference Meaning	Possible uom values
SFC	Height (above surface)	FT, M
MSL	Elevation (above mean sea level)	FT, M
W84	Ellipsoidal height (above the WGS 84 Ellipsoid) - currently not used for aeronautical operations	FT, M
STD	Pressure difference expressed as an equivalent vertical distance	FT, SM
OTHER: MY_VALUE	Exact meaning depends on what "MY_VALUE" says	FT, M

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:

Coded Value	uom (mandatory)
GND	FT
UNL	FT
FLOOR	OTHER
CEILING	OTHER

The "...Reference" attribute shall be left empty in this situation. However, even if another uom is used or even if the "...Reference" attribute gets a value, they should be ignored by a recipient application because they do not have any meaning in combination with these coded values.