The figure below shows the main AIXM 5 classes used for the touch down and lift off (TLOF) concept.
The TouchDownLiftOff class is used for defining a load bearing area on which a helicopter may touch down or lift-off.
A TLOF may be situated on a Runway (of type equal-to 'FATO') or directly at an AirportHeliport (i.e. the TLOF may form an integral part of the FATO or be situated apart).
A TLOF may be identified by its designator.
The extend of the TLOF may also be described ba the ElevatedSurface class. In addition, its dimension may be described by a length and a width.
A TLOF may have a centre point. In AIXM 5.1(.1), this is modelled with an association with role aimingPoint, using the ElevatedPoint class.
AIXM 5.1(.1) issue
The aiming point is not the same position as the centre point of a TLOF. According to ICAO Annex 14, Volume II,
The geographical coordinates of the geometric centre of the TLOF and/or of each threshold of the FATO (where appropriate) shall be measured and reported to the aeronautical information services authority
An aiming point marking should be provided at a heliport where it is necessary for a pilot to make an approach to a particular point before proceeding to the TLOF.
The aiming point marking shall be located within the FATO.
The name of the relationship hasAimingPoint and the property TLOF.aimingPoint are not correct, as they were intended to model the centre of the TLOF, not the "aiming point".A change proposal is discussed in the AIXM CCB, see issue AIXM-268
The surface material and strengths of a TLOF is modelled by using an association with the SurfaceCharacteristics class (see also Surface Characteristics [SCH]).