IFC geometrical concepts

IFC is a standard with a rich set of geometric primitives, with core concepts defined in the geometry, presentation, material, topology and profile resource subschemas. But also outside of these modules, there are various concepts that have geometrical implications such as openings and space boundary relationships in the productextension subschema. This listing below tries to provide a comprehensive overview broken down per the categories below:

Relational; typically IfcRelationship between elements subtypes, but also clearance geometry which is a potential additional shape representation identifier
Product-level; resources applicable the IfcProduct such as material-based styling; openings
Item-level; the individual items that make up representation of the element; which are in turn potentially affected by product-level features such as openings and styling
Context; provides additional information on individual items such as precision, target scale and semantic use of the geometry
Geo-referencing;

Prevalence is measured as a text-based scan through a set of 1000 files (500 for novel constructs). The number indicates percentage of model featuring this entity; and then the average number of instances for those models that contain any. See second table for more details.

Level Concept Prevalence Notes Related terms from ISO / OGC (SFA, GeoSPARQL, GML, …)

Level	Concept	Prevalence	Notes	Related terms from ISO / OGC (SFA, GeoSPARQL, GML, …)
Relational	Space boundaries (connection geometry)	12.6% (1676)	`IfcRelSpaceBoundary`: Surfaces representing adjacencies between spaces and building elements primarily intended for thermal simulation.	`geof:sfTouches`
Relational	Interferences	1.4% (3)	`IfcRelInterferesElements`: Primarily intended for modelling clash detection outcomes and over- and underpasses in infrastructure models. Likely has seen little use in practice.	`ST_Intersects`
Relational	Metric constraints	0%	`IfcMetric`: distances, parametric expressions between properties and geometry; supported in the schema (has limitations on indexing into unordered aggregates) and rarely used in practice.	`ST_Distance` `owl:sameAs` `rdfs:seeAlso`
Relational	Clearance geometry	0%	`IfcShapeRepresentation[RepresentationIdentifier='Clearance']`: Dedicated shapes encoding minimum clearances (e.g., around equipment, maintenance envelopes, door swings); a separate representation with its own context
Product	Relative positioning	97.6% (1298)	`IfcLocalPlacement` provides hierarchical local coordinate systems for products. Informal proposition are in place to define a placement hierarchy that follows the project decomposition structure	gml:AffineCS
Product	Linear positioning	4.8% (527)	`IfcLinearPlacement` provides linear positioning that derives an orientation and positioning frame from a length measure along an (IfcAlignment) curve	ISO 19148:2021
Product	Grid placement	0%	`IfcGridPlacement` provides placement by means of the computed intersection between two grid axes with underlying curve geometry
Product	Openings	49.1% (168)	`IfcRelVoidsElements` + `IfcOpeningElement`/`IfcVoidingFeature` models a subtractive boolean operation applied to host elements (walls, slabs, etc.); in principle extensible to other feature kinds such as `IfcProjectionElement`. In IFC2X3 CoordinationView these were to be processed by the importer. In IFC4 ReferenceView, in principle, these are precomputed at export time.	`ST_Difference`
Product	Material association	71% (324)	`IfcRelAssociatesMaterial`, layered / list / constituent / profile-based Links product geometry to material models: layered elements, material lists, constituents, profile-based materials. Especially layered materials (IfcMaterialLayerSet[Usage]) have geometric implications because it semantically slices the geometry by means of planes defined from offsets along a reference curve (for vertical elements) or along the extrusion direction.
Product	Parametric geometry as geometry-related semantics via property sets (e.g. `Pset_MechanicalFastenerBolt`, `IfcWindowLiningProperties`, `Pset_ProvisionForVoid`, most quantities `Qto_*`)	29% (73)	Semantic parameters that can be used to drive or reconstruct geometry when explicit representations are absent.
Product	Product-level composite curves defining the 'business-logic' of parametric alignment segment nesting within a curve	4.2% (504)	Decomposition of IfcAlignment into IfcAlignmentSegment with parametric geometry (typically redundant with item-level curves).	OGC LandInfra/InfraGML alignment model
Product	Aggregation of product into assemblies: composition (`IfcElementAssembly`) / nesting (ports, alignment segments)	96.9% (29)	Product-level aggregation (e.g., ports, sub-elements, segments)
Item	Basic 0D/1D/2D items: points, curves, faces (IfcCartesianPoint, IfcPolyline, IfcCompositeCurve, IfcFace)	99.2% (34194)	Simple representations or underlying points, polylines, composite curves (including circular/arc segments), and faces used in higher-level geometry.	SFA: Point, LineString, MultiLineString, Polygon; GML: gml:Point, gml:Curve, gml:LineString
Item	Profiles: parametric (e.g I shape) / arbitrary / derived / composite (IfcProfileDef, IfcArbitraryProfileDef, IfcDerivedProfileDef, IfcCompositeProfileDef) [Note: not strictly an independent IfcRepresentationItem]	76.9% (763)	Cross-section definitions used by sweeps and structural members; parametric catalog profiles and arbitrary section shapes.
Item	Annotation geometry: text literals, hatches and fills	11.6% (101)	`IfcTextLiteral`, `IfcFillAreaStyle` etc. 2D annotation geometry, symbolization and fills; mostly for documentation, not primary 3D model.
Item	Explicit (BRep) representations of 3D faceted solids and surfaces	43.9% (876)	`IfcFacetedBrep`	SFA: PolyhedralSurface
Item	Explicit (BRep) representations of 3D curved solids and surfaces	6.5% (112)	Advanced BRep: with (e.g) NURBS faces (IfcAdvancedFace)
Item	Explicit mesh/polyhedron representations (called tessellation)	12.6% (163)	`IfcTessellatedFaceSet` `IfcTriangulatedFaceSet` `IfcPolygonalFaceSet` Indexed face sets representing triangulated or polygonal surfaces/solids; compact storage via point lists and indices.	SFA: PolyhedralSurface gml:TriangulatedSurface
Item	TIN / surface models	2% (1)	`IfcTriangulatedIrregularNetwork` Specialized tessellated representation for terrain and horizontal surfaces (single f(X,Y)→Z for any X,Y); typical for infrastructure and terrain models.	gml:Tin
Item	Swept / procedural solids: extrusion, revolution, sweeps along curve (disk, arbitrary, fixed reference), sectioned solids and surfaces	75.5% (703)	3D solids defined by sweeping profiles along vectors or curves: straight/linear extrusions, revolutions, sweeps along curves (swept disk, arbitrary orientation, fixed reference), tapered forms, sectioned solids/surfaces. Note that this is a fairly large category with many items and combinatorial possibilities lumped into one.
Item	Axis geometry	47.6% (321)	Bounded curve geometries as (often) a secondary representation describing the longitudinal direction of a solid or element. Material layer sets are defined relative to this axis.
Item	Boolean operations (CSG trees)	30.6% (122)	Solids defined as binary trees combining operands via union, intersection, difference; often nested and require recursive traversal.	SFA: ST_Union, ST_Intersection, ST_Difference
Item	CSG primitives	1.7% (8)	`IfcCsgPrimitive3D` Are only part of the CSG expressions, but include things like cylinder and sphere which are 'harder' to produce otherwise (as revolutions)
Item	Surface-only geometry for connection / space boundaries and advanced face surfaces (extrusion, revolution, bounded, NURBS)	31.8% (1521)	Surfaces used without explicit solid closure, e.g. for space boundaries or analytical/architectural surfaces.	gml:Surface
Item	Styling by item as a whole (IfcStyledItem, IfcSurfaceStyle, IfcCurveStyle)	68.3% (1458)	Assignment of colors, line styles to geometric items or tessellated faces as a whole
Item	Granular styling by sub-item (IfcSurfaceStyle, IfcIndexedColourMap)	-	E.g the application of a different color to a face within a shell, or per-face colors in tesselations using IfcIndexedColourMap, or retaining distinct surface styles after applying boolean operations
Item	Styling by item: Textures	0.8% (2)	`IfcSurfaceStyleWithTextures` and three subtypes of `IfcSurfaceTexture` for a specialization of the above with texture as a binary blob, a compressed inline literal or an external URI reference. The complexity of supporting textures is mostly related to handling and defining texture coordinates (UV) on the procedural geometries (such as sweeps and booleans). Most likely `IfcReparametrisedCompositeCurveSegment` is also intended for supporting textures so that the U space of the profile prior to extrusion can be controlled.
Item	Mapped items / reusable definitions	46.3 1247	`IfcMappedItem` and type object occurrences: Instancing and transformations: one definition, multiple placed occurrences with transforms. In IFC2X3 it was not allowed to re-use the full representation as a whole, later in IFC4 that is possible.
Item	(Non-)uniform scaling and mirroring	-	`IfcMappedItem` is also the only mechanism in IFC that supports scaling and mirroring.
Item	Composite representations (IfcGeometricSet, IfcCompositeProfileDef, multiple shape representation items)	51.4% (231)	Sets of curves, surfaces, and points combined into a single representation; multi-rep shape representations for one object.	gml:MultiCurve, gml:MultiSurface, gml:CompositeCurve, gml:CompositeSurface
Item	Topological representations for analytical structures	0.3% (760)	`IfcEdge` `IfcVertex` `IfcFace` `IfcTopologyRepresentation` Explicit topology (vertices, edges, faces) as top-level items; often used in structural/analytical models.
Item	Point clouds and coverages	0.2% (1)	`IfcShapeRepresentation` + `IfcCartesianPointList3D`
Context	Representation context (IfcGeometricRepresentationContext): identifier, target scale, clearance geometries, subcontexts	-	Defines modeling context: dimensionality, precision, world coordinate system, true north, representation identifiers; may host clearance contexts.
Context	Tolerance and precision settings	-	`IfcGeometricRepresentationContext.Precision` Model-level precision and tolerances that influence how geometry is interpreted and when elements are considered coincident. Only linear tolerance, no angular setting and no local (e.g per-vertex) possibilities.
Georeferencing	Local engineering CRS (IfcGeometricRepresentationContext.WorldCoordinateSystem, true north)	-	Defines the local engineering coordinate system
Georeferencing	Map/projection-based georeferencing incl. vertical reference	12.4% (2)	`IfcMapConversion`, `IfcProjectedCRS` Conversion from local model coordinates to a map/projected CRS; explicit link to EPSG or other CRS identifiers; since IFC4/IFC4x3.	ISO 19111 referencing by coordinates and transformations; SFA: SRID on geometries.
Georeferencing	Postal address	38.6% (3)	`IfcPostalAddress`, `Pset_Address`
Georeferencing	IfcSite reference coordinates	-	`IfcSite` has DMS reference coords for latitude and longitude

Relational

Space boundaries (connection geometry)

12.6% (1676)

IfcRelSpaceBoundary: Surfaces representing adjacencies between spaces and building elements primarily intended for thermal simulation.

geof:sfTouches

Relational

Interferences

1.4% (3)

IfcRelInterferesElements: Primarily intended for modelling clash detection outcomes and over- and underpasses in infrastructure models. Likely has seen little use in practice.

ST_Intersects

Relational

Metric constraints

IfcMetric: distances, parametric expressions between properties and geometry; supported in the schema (has limitations on indexing into unordered aggregates) and rarely used in practice.

ST_Distance owl:sameAs rdfs:seeAlso

Relational

Clearance geometry

IfcShapeRepresentation[RepresentationIdentifier='Clearance']: Dedicated shapes encoding minimum clearances (e.g., around equipment, maintenance envelopes, door swings); a separate representation with its own context

Product

Relative positioning

97.6% (1298)

IfcLocalPlacement provides hierarchical local coordinate systems for products. Informal proposition are in place to define a placement hierarchy that follows the project decomposition structure

gml:AffineCS

Product

Linear positioning

4.8% (527)

IfcLinearPlacement provides linear positioning that derives an orientation and positioning frame from a length measure along an (IfcAlignment) curve

ISO 19148:2021

Product

Grid placement

IfcGridPlacement provides placement by means of the computed intersection between two grid axes with underlying curve geometry

Product

Openings

49.1% (168)

IfcRelVoidsElements + IfcOpeningElement/IfcVoidingFeature models a subtractive boolean operation applied to host elements (walls, slabs, etc.); in principle extensible to other feature kinds such as IfcProjectionElement. In IFC2X3 CoordinationView these were to be processed by the importer. In IFC4 ReferenceView, in principle, these are precomputed at export time.

ST_Difference

Product

Material association

71% (324)

IfcRelAssociatesMaterial, layered / list / constituent / profile-based Links product geometry to material models: layered elements, material lists, constituents, profile-based materials. Especially layered materials (IfcMaterialLayerSet[Usage]) have geometric implications because it semantically slices the geometry by means of planes defined from offsets along a reference curve (for vertical elements) or along the extrusion direction.

Product

Parametric geometry as geometry-related semantics via property sets (e.g. Pset_MechanicalFastenerBolt, IfcWindowLiningProperties, Pset_ProvisionForVoid, most quantities Qto_*)

29% (73)

Semantic parameters that can be used to drive or reconstruct geometry when explicit representations are absent.

Product

Product-level composite curves defining the 'business-logic' of parametric alignment segment nesting within a curve

4.2% (504)

Decomposition of IfcAlignment into IfcAlignmentSegment with parametric geometry (typically redundant with item-level curves).

OGC LandInfra/InfraGML alignment model

Product

Aggregation of product into assemblies: composition (IfcElementAssembly) / nesting (ports, alignment segments)

96.9% (29)

Product-level aggregation (e.g., ports, sub-elements, segments)

Item

Basic 0D/1D/2D items: points, curves, faces (IfcCartesianPoint, IfcPolyline, IfcCompositeCurve, IfcFace)

99.2% (34194)

Simple representations or underlying points, polylines, composite curves (including circular/arc segments), and faces used in higher-level geometry.

SFA: Point, LineString, MultiLineString, Polygon; GML: gml:Point, gml:Curve, gml:LineString

Item

Profiles: parametric (e.g I shape) / arbitrary / derived / composite (IfcProfileDef, IfcArbitraryProfileDef, IfcDerivedProfileDef, IfcCompositeProfileDef) [Note: not strictly an independent IfcRepresentationItem]

76.9% (763)

Cross-section definitions used by sweeps and structural members; parametric catalog profiles and arbitrary section shapes.

Item

Annotation geometry: text literals, hatches and fills

11.6% (101)

IfcTextLiteral, IfcFillAreaStyle etc. 2D annotation geometry, symbolization and fills; mostly for documentation, not primary 3D model.

Item

Explicit (BRep) representations of 3D faceted solids and surfaces

43.9% (876)

IfcFacetedBrep

SFA: PolyhedralSurface

Item

Explicit (BRep) representations of 3D curved solids and surfaces

6.5% (112)

Advanced BRep: with (e.g) NURBS faces (IfcAdvancedFace)

Item

Explicit mesh/polyhedron representations (called tessellation)

12.6% (163)

IfcTessellatedFaceSet IfcTriangulatedFaceSet IfcPolygonalFaceSet Indexed face sets representing triangulated or polygonal surfaces/solids; compact storage via point lists and indices.

SFA: PolyhedralSurface gml:TriangulatedSurface

Item

TIN / surface models

2% (1)

IfcTriangulatedIrregularNetwork Specialized tessellated representation for terrain and horizontal surfaces (single f(X,Y)→Z for any X,Y); typical for infrastructure and terrain models.

gml:Tin

Item

Swept / procedural solids: extrusion, revolution, sweeps along curve (disk, arbitrary, fixed reference), sectioned solids and surfaces

75.5% (703)

3D solids defined by sweeping profiles along vectors or curves: straight/linear extrusions, revolutions, sweeps along curves (swept disk, arbitrary orientation, fixed reference), tapered forms, sectioned solids/surfaces. Note that this is a fairly large category with many items and combinatorial possibilities lumped into one.

Item

Axis geometry

47.6% (321)

Bounded curve geometries as (often) a secondary representation describing the longitudinal direction of a solid or element. Material layer sets are defined relative to this axis.

Item

Boolean operations (CSG trees)

30.6% (122)

Solids defined as binary trees combining operands via union, intersection, difference; often nested and require recursive traversal.

SFA: ST_Union, ST_Intersection, ST_Difference

Item

CSG primitives

1.7% (8)

IfcCsgPrimitive3D Are only part of the CSG expressions, but include things like cylinder and sphere which are 'harder' to produce otherwise (as revolutions)

Item

Surface-only geometry for connection / space boundaries and advanced face surfaces (extrusion, revolution, bounded, NURBS)

31.8% (1521)

Surfaces used without explicit solid closure, e.g. for space boundaries or analytical/architectural surfaces.

gml:Surface

Item

Styling by item as a whole (IfcStyledItem, IfcSurfaceStyle, IfcCurveStyle)

68.3% (1458)

Assignment of colors, line styles to geometric items or tessellated faces as a whole

Item

Granular styling by sub-item (IfcSurfaceStyle, IfcIndexedColourMap)

E.g the application of a different color to a face within a shell, or per-face colors in tesselations using IfcIndexedColourMap, or retaining distinct surface styles after applying boolean operations

Item

Styling by item: Textures

0.8% (2)

IfcSurfaceStyleWithTextures and three subtypes of IfcSurfaceTexture for a specialization of the above with texture as a binary blob, a compressed inline literal or an external URI reference. The complexity of supporting textures is mostly related to handling and defining texture coordinates (UV) on the procedural geometries (such as sweeps and booleans). Most likely IfcReparametrisedCompositeCurveSegment is also intended for supporting textures so that the U space of the profile prior to extrusion can be controlled.

Item

Mapped items / reusable definitions

46.3 1247

IfcMappedItem and type object occurrences: Instancing and transformations: one definition, multiple placed occurrences with transforms. In IFC2X3 it was not allowed to re-use the full representation as a whole, later in IFC4 that is possible.

Item

(Non-)uniform scaling and mirroring

IfcMappedItem is also the only mechanism in IFC that supports scaling and mirroring.

Item

Composite representations (IfcGeometricSet, IfcCompositeProfileDef, multiple shape representation items)

51.4% (231)

Sets of curves, surfaces, and points combined into a single representation; multi-rep shape representations for one object.

gml:MultiCurve, gml:MultiSurface, gml:CompositeCurve, gml:CompositeSurface

Item

Topological representations for analytical structures

0.3% (760)

IfcEdge IfcVertex IfcFace IfcTopologyRepresentation Explicit topology (vertices, edges, faces) as top-level items; often used in structural/analytical models.

Item

Point clouds and coverages

0.2% (1)

IfcShapeRepresentation + IfcCartesianPointList3D

Context

Representation context (IfcGeometricRepresentationContext): identifier, target scale, clearance geometries, subcontexts

Defines modeling context: dimensionality, precision, world coordinate system, true north, representation identifiers; may host clearance contexts.

Context

Tolerance and precision settings

IfcGeometricRepresentationContext.Precision Model-level precision and tolerances that influence how geometry is interpreted and when elements are considered coincident. Only linear tolerance, no angular setting and no local (e.g per-vertex) possibilities.

Georeferencing

Local engineering CRS (IfcGeometricRepresentationContext.WorldCoordinateSystem, true north)

Defines the local engineering coordinate system

Georeferencing

Map/projection-based georeferencing incl. vertical reference

12.4% (2)

IfcMapConversion, IfcProjectedCRS Conversion from local model coordinates to a map/projected CRS; explicit link to EPSG or other CRS identifiers; since IFC4/IFC4x3.

ISO 19111 referencing by coordinates and transformations; SFA: SRID on geometries.

Georeferencing

Postal address

38.6% (3)

IfcPostalAddress, Pset_Address

Georeferencing

IfcSite reference coordinates

IfcSite has DMS reference coords for latitude and longitude

Methodology for testing prevalence.

Testing the prevalence of entities in existing sample models is often criticized because it is hard to find a body of representative IFC models (every tool and vendor cater to their own niche) and prevalence does not equate value. Lastly, per definition, an approach like this is backwards looking and not forward looking. At the same time, the investigation into 3D GeoSPARQL is explicitly use-case driven.

A random sample of 1_000 files (dataset 1) is taken from a collection of models for which it is allowed to publish in aggregate form. For novel concepts such as (IfcLinearPlacement, IfcRelInterferes, IfcAlignmentSegment, …) a 500 file sample of IFC4.3 files is selected (dataset 2; because the amount of models for older schema versions is larger). It is possible that there are invalid or partial files in this sample, there is no verdict on quality, correctness or applicability of these files. Example files from the specification can have a distorting impact on the sample, also there is no filtering for (near-) duplicates. Some values such as IfcSite.RefLatitude -Longitude are well known to contain placeholder values on older exports such as based on the headquarters of the software vendor.

For performance reasons we don’t use an IFC parser for testing the prevalence of entities. Therefore, in theory it is possible that we match fragments of comments or human readable text in the model, but this is given the patterns highly unlikely. We use ripgrep rg to query the model as text files on disk. Due to the text-based parsing we cannot fully asses the context in which entities are used - in particular entities like IfcRelAggregates are discussed here in an building-element context, but are also found in the spatial structure and therefore the prevalence is reported higher.

Query string	Dataset index	% of models	avg nr instances when used
IFCRELAGGREGATES	1	96.9	29
IFCCARTESIANPOINT	1	99.2	34194
PROFILEDEF	1	76.9	763
IFCANNOTATIONFILLAREA\|IFCANNOTATIONSURFACE\|IFCDRAUGHTINGCALLOUT\|IFCFILLAREASTYLEHATCHING\|IFCTEXTLITERAL	1	11.6	101
IFCFACETEDBREP	1	43.9	876
IFCADVANCEDBREP	1	6.5	112
IFCTESSELLATEDFACESET\|IFCTRIANGULATEDFACESET\|IFCPOLYGONALFACESET	1	12.6	163
IFCTRIANGULATEDIRREGULARNETWORK	2	2	1
IFCEXTRUDEDAREASOLID\|IFCREVOLVEDAREASOLID\|IFCFIXEDREFERENCESWEPTAREASOLID\|IFCSURFACECURVESWEPTAREASOLID\|IFCDIRECTRIXDERIVEDREFERENCESWEPTAREASOLID	1	75.5	703
IFCSHAPEREPRESENTATION.*'Axis	1	47.6	321
IFCBOOLEANRESULT\|IFCBOOLEANCLIPPINGRESULT	1	30.6	122
IFCBLOCK\|IFCRECTANGULARPYRAMID\|IFCRIGHTCIRCULARCONE\|IFCRIGHTCIRCULARCYLINDER\|IFCSPHERE	1	1.7	8
IFCCURVEBOUNDEDPLANE\|IFCCURVEBOUNDEDSURFACE\|IFCBSPLINESURFACE\|IFCCYLINDRICALSURFACE\|IFCPLANE\(\|IFCSPHERICALSURFACE\|IFCTOROIDALSURFACE\|IFCSURFACEOFLINEAREXTRUSION\|IFCSURFACEOFREVOLUTION	1	31.8	1521
IFCSTYLEDITEM\|IFCSURFACESTYLE\|IFCCURVESTYLE	1	68.3	1458
-
IFCBLOBTEXTURE\|IFCIMAGETEXTURE\|IFCPIXELTEXTURE	1	0.8	2
IFCMAPPEDITEM	1	46.3	1247
-
IFCGEOMETRICSET\|IFCCOMPOSITEPROFILEDEF\|IFCGEOMETRICCURVESET\|IFCSHAPEREPRESENTATION\(.\(.,	1	51.4	231
IFCTOPOLOGYREPRESENTATION	1	0.3	760
IFCSHAPEREPRESENTATION.*'Point	1	0.2	1
-
-
-
IFCPROJECTEDCRS\|IFCGEOGRAPHICCRS\|IFCMAPCONVERSION\|IFCRIGIDOPERATION	2	12.4	2
IFCPOSTALADDRESS\|Pset_Address	1	38.6	3
-