# Ifc Syntax Georeferencing > Use when you need to anchor an IFC model to real-world geographic coordinates : declare where the local project origin sits on Earth, rotate the model onto a map grid, pick between coarse site latitude/longitude and a precise projected CRS, or place infrastructure objects along an alignment by station and offset. Prevents treating the coarse IfcSite latitude as precise georeferencing, applying the map-conversion translation and rotation in the wrong order, mixing signs in a compound plane angle, targeting a map conversion at a non-projected CRS, emitting IFC4 or IFC4.3 georeferencing entities into an IFC2x3 file, and authoring geometry at raw projected coordinates. Covers IfcSite RefLatitude / RefLongitude / RefElevation, IfcCompoundPlaneAngleMeasure, IfcMapConversion, IfcProjectedCRS, IfcCoordinateReferenceSystem, IfcCoordinateOperation, IfcMapConversionScaled, the IFC4.3 IfcLinearPlacement family, the LoGeoRef levels, and the IFC2x3 / IFC4 / IFC4.3 version differences. Keywords: IFC georeferencing, IfcMapCo - Skill: `impertio-studio/ifc-syntax-georeferencing` (Agent Skill, multi-file: 4 files) - Install (CLI): `npx skillmds@latest add impertio-studio/ifc-syntax-georeferencing` - Raw SKILL.md: https://api.skillmd.com/api/skills/impertio-studio/ifc-syntax-georeferencing/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: AI & ML - License: MIT - Author: Impertio-Studio (https://skillmd.com/u/impertio-studio) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/impertio-studio/ifc-syntax-georeferencing --- # IFC Syntax : Georeferencing IFC geometry is authored in a flat, metre-scale local engineering coordinate system. Georeferencing is the separate, additive layer that tells a consumer where that local origin sits on Earth and how it is rotated onto a map grid. This skill covers the three generations of georeferencing mechanism, the entities they use, the transformation order, and the version availability. Legacy site coordinates exist in IFC2x3, IFC4, and IFC4.3. Precise georeferencing (`IfcMapConversion`, `IfcProjectedCRS`) is IFC4 and later. Linear placement is IFC4.3 only. The Version Differences section is load-bearing. ## Quick Reference ### The three mechanisms ``` Coarse legacy (IFC2x3, IFC4, IFC4.3) IfcSite.RefLatitude / RefLongitude / RefElevation One anchor point, WGS84 angular, NO rotation. Sun-shading accuracy only. Precise (IFC4, IFC4.3) IfcMapConversion + IfcProjectedCRS Full scale + rotation + translation into a projected map CRS. Survey accuracy. Linear (IFC4.3 only) IfcLinearPlacement + IfcAxis2PlacementLinear + IfcPointByDistanceExpression Positions infrastructure objects by station and offset along an alignment curve. ``` ALWAYS use `IfcMapConversion` plus `IfcProjectedCRS` when survey, GIS hand-off, or infrastructure coordination accuracy is needed. NEVER rely on `IfcSite` latitude and longitude for precise placement : the specification states they are not meant to replace precise georeferencing. ### Legacy : IfcSite reference coordinates `IfcSite` carries three optional reference attributes, present in every IFC version : - `RefLatitude` : `OPTIONAL IfcCompoundPlaneAngleMeasure`. World latitude at the reference point, relative to WGS84. - `RefLongitude` : `OPTIONAL IfcCompoundPlaneAngleMeasure`. World longitude, relative to WGS84. - `RefElevation` : `OPTIONAL IfcLengthMeasure`. Datum elevation relative to sea level. `IfcCompoundPlaneAngleMeasure` is verified verbatim : ``` TYPE IfcCompoundPlaneAngleMeasure = LIST [3:4] OF INTEGER; ``` Element 1 is degrees, 2 is minutes, 3 is seconds, optional 4 is millionths of a second. ALL non-zero components must share one sign. A southern latitude of 51 degrees 30 minutes 26 seconds is `(-51, -30, -26)`, NEVER `(-51, 30, 26)`. This mechanism locates one point and carries no rotation. The model's bearing onto the map is unknown from the site point alone. ### Precise : IfcMapConversion `IfcMapConversion` is a new entity in IFC4. It expresses the full rigid transform from the local engineering CRS into a projected map CRS. Verified verbatim (IFC4.3) : ``` ENTITY IfcMapConversion SUPERTYPE OF (ONEOF (IfcMapConversionScaled)) SUBTYPE OF (IfcCoordinateOperation); Eastings : IfcLengthMeasure; Northings : IfcLengthMeasure; OrthogonalHeight : IfcLengthMeasure; XAxisAbscissa : OPTIONAL IfcReal; XAxisOrdinate : OPTIONAL IfcReal; Scale : OPTIONAL IfcReal; WHERE TargetCRSOnlyProjected : 'IFC4X3_DEV.IFCPROJECTEDCRS' IN TYPEOF(SELF\IfcCoordinateOperation.TargetCRS); END_ENTITY; ``` - `Eastings`, `Northings`, `OrthogonalHeight` : required. The translation of the local origin in the map CRS. - `XAxisAbscissa`, `XAxisOrdinate` : optional. The local X axis expressed as a 2D direction in the map easting/northing plane. Together they encode the grid bearing. - `Scale` : optional, defaults to `1.0`. A single uniform scale factor. - `SourceCRS`, `TargetCRS` : inherited from `IfcCoordinateOperation`. `SourceCRS` is the project's `IfcGeometricRepresentationContext`; `TargetCRS` must be an `IfcProjectedCRS` (the `TargetCRSOnlyProjected` rule enforces it). The IFC4 baseline definition is identical except it has no `WHERE` rule and no `IfcMapConversionScaled` subtype. ### The transformation order The map conversion applies, in this exact sequence : ``` 1. SCALE : multiply x, y, z by Scale (uniform). 2. ROTATE : anti-clockwise about the Z axis by theta = arctan(XAxisOrdinate / XAxisAbscissa). 3. TRANSLATE : add (Eastings, Northings, OrthogonalHeight). ``` ALWAYS compose the matrix as `T * R * S` (translation outermost). NEVER translate before rotating, and NEVER rotate before scaling : the order is load-bearing and any other order mis-places every element. ### Precise : IfcProjectedCRS `IfcProjectedCRS` describes the target projected CRS : datum, projection, zone. A new entity in IFC4. Verified verbatim (IFC4.3) : ``` ENTITY IfcProjectedCRS SUBTYPE OF (IfcCoordinateReferenceSystem); VerticalDatum : OPTIONAL IfcIdentifier; MapProjection : OPTIONAL IfcIdentifier; MapZone : OPTIONAL IfcIdentifier; MapUnit : OPTIONAL IfcNamedUnit; WHERE MapUnitIsLength : NOT(EXISTS(SELF.MapUnit)) OR (SELF.MapUnit.UnitType = IfcUnitEnum.LENGTHUNIT); END_ENTITY; ``` `Name`, `Description`, `GeodeticDatum` are inherited from `IfcCoordinateReferenceSystem`. Typical values : `Name = 'EPSG:25832'`, `GeodeticDatum = 'ETRS89'`, `MapProjection = 'UTM'`, `MapZone = '32N'`. The `MapUnitIsLength` rule requires `MapUnit`, if present, to be a length unit. ### IFC4.3 : linear placement `IfcLinearPlacement` positions an object by station and offset along an alignment curve, instead of by an absolute Cartesian triple. It is IFC4.3 only. Verified verbatim : ``` ENTITY IfcLinearPlacement SUBTYPE OF (IfcObjectPlacement); RelativePlacement : IfcAxis2PlacementLinear; CartesianPosition : OPTIONAL IfcAxis2Placement3D; END_ENTITY; ``` `RelativePlacement` is an `IfcAxis2PlacementLinear` whose inherited `Location` is an `IfcPointByDistanceExpression` (carrying `DistanceAlong`, the station, plus lateral and vertical offsets and a `BasisCurve`). `CartesianPosition` is the optional Cartesian fallback for consumers that do not implement linear referencing. ALWAYS write `CartesianPosition` alongside an `IfcLinearPlacement` : a geometry-only consumer that lacks linear referencing cannot place the object without it. ### LoGeoRef levels (informative) LoGeoRef (Level of Georeferencing) is an informative academic classification, NOT part of the IFC schema. Levels 10 to 50, higher is better. Level 20 is `IfcSite` latitude/longitude; level 50 is `IfcMapConversion` plus `IfcProjectedCRS`. Level 50 requires IFC4 or later. NEVER cite LoGeoRef as a buildingSMART normative rule. ### Version differences (compact) | Entity / mechanism | IFC2x3 | IFC4 | IFC4.3 | |--------------------|--------|------|--------| | `IfcSite` Ref latitude / longitude / elevation | yes | yes | yes | | `IfcCompoundPlaneAngleMeasure` | yes | yes | yes | | `IfcMapConversion`, `IfcProjectedCRS` | no | yes | yes | | `IfcCoordinateOperation`, `IfcCoordinateReferenceSystem` | no | yes | yes | | `IfcMapConversion` `WHERE` rule + WKT inverse | no | no | yes | | `IfcMapConversionScaled` | no | no | yes | | `IfcLinearPlacement` family | no | no | yes | | LoGeoRef 50 achievable | no | yes | yes | ## Decision Trees ### Coarse or precise georeferencing? ``` How accurate must the model's real-world position be? | +-- Sun-shading, rough context, no survey hand-off | --> IfcSite.RefLatitude / RefLongitude / RefElevation is enough. | Coarse, all IFC versions, no rotation. | +-- Survey accuracy, GIS hand-off, infrastructure coordination | --> IfcMapConversion + IfcProjectedCRS. IFC4 or later only. | +-- The file must be IFC2x3 and still georeferenced --> IfcSite lat/long is the best available. Precise entities do NOT exist in IFC2x3. Add TrueNorth on the representation context for an approximate bearing. ``` ### Which target CRS? ``` What is IfcMapConversion.TargetCRS? | +-- ALWAYS an IfcProjectedCRS (a linear, projected map CRS). | The TargetCRSOnlyProjected WHERE rule enforces this. | +-- NEVER an IfcGeographicCRS or any non-projected CRS : the rule rejects it, and an angular CRS has no metric eastings/northings. ``` ### Cartesian or linear placement? ``` How is the object naturally located? | +-- By an X/Y/Z position in the building | --> IfcLocalPlacement (see ifc-syntax-geometry-placement). | +-- By a structural grid intersection | --> IfcGridPlacement. | +-- By a station and offset along a road / rail / bridge alignment --> IfcLinearPlacement (IFC4.3 only). ALWAYS add the CartesianPosition fallback. ``` ## Patterns ### Pattern : Use IfcSite coordinates only for coarse context ALWAYS treat `IfcSite.RefLatitude`, `RefLongitude`, and `RefElevation` as a coarse anchor point suitable for sun-shading and rough siting. NEVER present the site latitude and longitude as precise georeferencing : the `IfcSite` specification states they are not meant to replace it, they carry no grid rotation, and they mix an angular datum with a flat Cartesian model. ### Pattern : Keep compound plane angle signs consistent ALWAYS make every non-zero component of an `IfcCompoundPlaneAngleMeasure` carry the same sign. A western longitude of 0 degrees 7 minutes 39 seconds is `(0, -7, -39)`. NEVER mix signs (`(-51, 30, 26)`) : the `WHERE` rule on the measure requires sign consistency and the buildingSMART validation service flags a violation. ### Pattern : Wire the map conversion to the representation context ALWAYS set `IfcMapConversion.SourceCRS` to the project's 3D model `IfcGeometricRepresentationContext` : the `IfcCoordinateReferenceSystemSelect` type permits it precisely because that context is the local engineering CRS. ALWAYS set `IfcMapConversion.TargetCRS` to an `IfcProjectedCRS`. There is no direct attribute on `IfcProject` for the map conversion : the link runs through the representation context acting as `SourceCRS`. See `ifc-syntax-geometry-placement` for the placement tree and the representation context. ### Pattern : Apply the transformation in scale-rotate-translate order ALWAYS apply the map conversion as scale first, then anti-clockwise Z rotation, then translation, and compose the 4x4 matrix as `T * R * S`. ALWAYS derive the rotation angle as `arctan(XAxisOrdinate / XAxisAbscissa)`. NEVER translate before rotating or rotate before scaling : the order is fixed by the specification and any other order mis-places every element. ### Pattern : Always express the grid bearing ALWAYS populate `XAxisAbscissa` and `XAxisOrdinate` when the model's local X axis is not parallel to map easting. They express the local X axis as a 2D direction in the map plane. NEVER omit the X-axis direction pair on a rotated model : when both are absent the consumer applies no rotation and the model is silently mis-oriented on the map. ### Pattern : Keep geometry near a local origin ALWAYS author building geometry near a local origin, with coordinates in the metre-to-kilometre range, and express the real-world offset only through `IfcMapConversion`. NEVER author geometry at raw projected eastings and northings (coordinates in the millions of metres) : large coordinate values destroy floating-point precision in downstream geometry kernels. ### Pattern : Provide the linear-placement fallback ALWAYS write `IfcLinearPlacement.CartesianPosition` (an `IfcAxis2Placement3D`) alongside the linear `RelativePlacement` when exporting an IFC4.3 infrastructure model. NEVER ship a linear placement without the Cartesian fallback : a consumer that does not implement linear referencing then cannot place the object at all. ### Pattern : Honour the version availability ALWAYS restrict `IfcMapConversion`, `IfcProjectedCRS`, `IfcCoordinateOperation`, and `IfcCoordinateReferenceSystem` to IFC4 and IFC4.3 files. ALWAYS restrict `IfcLinearPlacement`, `IfcAxis2PlacementLinear`, `IfcPointByDistanceExpression`, and `IfcMapConversionScaled` to IFC4.3 files. NEVER emit any of these into an IFC2x3 file : they do not exist in the IFC2x3 schema and the file fails validation. See `ifc-core-version-evolution`. ## Reference Links - `references/methods.md` : the complete verified EXPRESS definitions and attribute tables for every georeferencing entity, the abstract supertypes, the select type, and the full version-availability matrix. - `references/examples.md` : worked STEP scenarios : a legacy `IfcSite` reference point, a complete `IfcMapConversion` plus `IfcProjectedCRS` block, an `IfcLinearPlacement`, and reading a point through the transformation. - `references/anti-patterns.md` : the failure modes this skill prevents, each with the rule or version reason it violates. ### Official sources - IfcSite : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcSite.htm - IfcCompoundPlaneAngleMeasure : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcCompoundPlaneAngleMeasure.htm - IfcMapConversion : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcMapConversion.htm - IfcProjectedCRS : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcProjectedCRS.htm - IfcCoordinateReferenceSystem : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcCoordinateReferenceSystem.htm - IfcCoordinateOperation : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcCoordinateOperation.htm - IfcLinearPlacement : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcLinearPlacement.htm - IfcMapConversion (IFC4) : https://standards.buildingsmart.org/IFC/RELEASE/IFC4/ADD2_TC1/HTML/schema/ifcrepresentationresource/lexical/ifcmapconversion.htm ### Related skills - `ifc-syntax-geometry-placement` : the local placement tree and the `IfcGeometricRepresentationContext` that acts as the map-conversion `SourceCRS`. - `ifc-core-version-evolution` : which entities exist in which IFC version.