IFC spatial tree

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An example spatial tree. Orange classes belong to IFC context, blue classes belong to IfcSpatialElement, and green classes belong to IfcElement. Blue arrows represent an aggregation relationship, and green arrows represent a spatial containment relationship.

IFC data often, but not always, contains what is known as a spatial tree or spatial hierarchy. This is a tree of classes that inherit from IfcSpatialElement that describe the spatial organisation of a building. For example, it might describe that a site contains two buildings, and each building contains 5 storeys, and each storey contains 3 spaces.

As all IFC data must start from an IFC context, such as an IfcProject, the top level of the spatial tree is assigned to the IFC context. If the context is an IfcProjectLibrary, a spatial tree may not exist. This top level of the spatial tree may be any IfcSpatialElement, but is commonly an IfcSite class. The relationship between the top level of the spatial tree and the IFC context is known as an aggregation relationship, or as spatial (de)composition.

Within the spatial tree itself, a IfcSpatialElement subclass may then contain zero or more IfcSpatialElement subclasses. This builds up a spatial hierarchy. For example, IfcSite may contain multiple IfcBuilding. This relationship between IfcSpatialElement subclasses is also known as an aggregation relationship, or as spatial (de)composition.

Usually at the bottom of the spatial hierarchy, the spatial tree may end and contain actual physical built elements, such as walls and columns. Walls and columns are not IfcSpatialElement, but instead belong to IfcElement. To an end-user, a space containing a wall and column is similar to how a site contains a building, but in IFC, this is known as a spatial containment relationship.

A list of possible subclasses of IfcSpatialElement is shown below.

IFC Class Description
IfcExternalSpatialElement The external spatial element defines external regions at the building site. Examples include external air space around the building, a volume covered by earth or water around the building, or a neighbouring fire risk.
IfcBridge A Bridge is civil engineering works that affords passage to pedestrians, animals, vehicles, and services above obstacles or between two points at a height above ground.
IfcBuilding A building represents a structure that provides shelter for its occupants or contents and stands in one place.
IfcBridgePart In a bridge, IfcBridgePart represents the parts according to local practices (e.g. Superstructure, Substructure, Foundation).
IfcBuildingStorey The building storey has an elevation and typically represents a (nearly) horizontal aggregation of spaces that are vertically bound.
IfcSite A site is a defined area of land, possibly covered with water, on which the project construction is to be completed. A site may be used to erect, retrofit or turn down building(s), or for other construction related developments.
IfcSpace A space represents an area or volume bounded actually or theoretically. Spaces are areas or volumes that provide for certain functions within a building, like a room.
IfcSpatialZone A spatial zone is a non-hierarchical and potentially overlapping decomposition of the project under some functional consideration. A spatial zone might be used to represent a thermal zone, a construction zone, a lighting zone, a usable area zone.