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
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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, IfcMapConversion, IfcProjectedCRS, IfcCoordinateReferenceSystem, IfcCoordinateOperation, IfcMapConversionScaled, IfcSite, RefLatitude, RefLongitude, RefElevation, IfcCompoundPlaneAngleMeasure, Eastings, Northings, OrthogonalHeight, XAxisAbscissa, XAxisOrdinate, EPSG, UTM, WGS84, ETRS89, projected CRS, true north, IfcLinearPlacement, IfcAxis2PlacementLinear, IfcPointByDistanceExpression, station, chainage, LoGeoRef, my model is in the wrong place, model not on the map, how do I set coordinates, model rotated wrong, coordinates too large, IFC2x3, IFC4, IFC4.3.
license
MIT
compatibility
Designed for Claude Code. Requires IFC IFC2x3, IFC4, IFC4.3.
metadata
{"author":"OpenAEC-Foundation","version":"1.0"}
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 :
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.
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.
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.
