The AIXM model for "obstacles" takes a 2-tier approach:

first, there is the VerticalStructure class, which is defined as "All fixed (whether temporary or permanent) and mobile objects, or parts thereof that extend above the surface of the Earth". This includes obstacles, but also buildings and any other structures that are raised above the surface and that are relevant for aircraft operations. The definition of VerticalStructure has a second sentence that indicates: "Those vertical structures that are located on an area intended for the surface movement of aircraft or that extend above a defined surface intended to protect aircraft in flight are considered obstacles."
then, the association between VerticalStructure and ObstacleArea allows indicating precisely which vertical structures are considered obstacles in one or more obstacle data collection surfaces.

The UML class diagram below shows the main AIXM 5.1(.1) classes used to model vertical structures.

The VerticalStructure class contains high-level properties and associations, such as:

name - if applicable, the name under which the obstacle is known to the public
type - allowing to categorise the obstacles based on a predefined list (building, bridge, tree, terrain high-point, etc.)
group - indicating if the obstacle is part of a larger set of similar obstacles situated closely to each other
lighted - a yes/no indication if the obstacle is visually identified with light sources. This can be complemented with an indication of conformance to the ICAO standards (lightingICAOStandard) and details (position, colour, etc.) about each light elements, as explained in the vertical structure lighting diagram
markingICAOStandard - indicates if the obstacle is painted of marked with a flag or other visible element (not lighting), as specified in the ICAO SARPS.

Note

A change proposal is being discussed for AIXM 5.2 in order to align the model for obstacle marking with the model for lighting.

length, width and radius - provide the possibility to specify the overall dimensions of the obstacle projection on the surface of the Earth. For certain obstacle types, such as a pole with guy wires, the radius attribute can be used to indicate the circular area covered by the wires.

The physical details of an obstacle are modelled through the isMadeOf association with the VerticalStructurePart class. This allows coding the geometry of complex obstacles, made of several parts, such as buildings, power lines, etc. It included the following attributes and associations:

designator - if applicable, an official identification of the obstacle or its part
type - allows to provide a distinct type for the part, which can be specific and different from the parent obstacle type
constructionStatus - can be used to indicate if an obstacle is still in construction, scheduled for demolition, etc.
mobile - a yes/no indication that the part is not fix, but it can change shape and/or position
markingPattern, markingFirstColour, markingSecondColour - provides details about the obstacle visual marking elements

The actual horizontal and vertical extent of the obstacle are modelled with:

the association isRepresentdAs with VerticalStructurePartGeometry, which allows specifying a Point, Curve (line) of Surface (polygon) type of geometry, which in fact corresponds to the projection oft the part geometry on a horizontal plane located at the top of the ObstaclePart;
the verticalExtent attribute, which corresponds to the height of the part (for obstacles sitting on the ground) or to the difference between the to and the bottom of the VerticalStructurePart (for obstacles that have parts sitting off the ground, such as sections of a bridge;

The VerticalStructurePart also has a speclalisation association with the PropertiesWithSchedule class. This means that a VerticalStructurePart may have a associated timetable, expressed with one or more Timesheet, For moblie obstacles or for obstacles that have a mobile part (such as cranes, bridges, etc.) this indicates the time when that obstacle part actually exists in the form and at the location detailed by the VerticalStructurePart properties.

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