| name | ifc-syntax-geometry-placement |
| description | Use when you need to position an IFC product in space, build or read the placement tree, or resolve an element to world coordinates : the door placed relative to its wall, the wall relative to its storey, the storey relative to the building, and so on up to the project world coordinate system. Prevents the placement errors that put elements in the wrong spot : a 3D placement chained to a 2D parent, a circular PlacementRelTo reference, supplying only one of Axis and RefDirection, forgetting to multiply the whole chain, and confusing the local geometry origin with the world position. Covers IfcObjectPlacement, IfcLocalPlacement (PlacementRelTo chaining and RelativePlacement), IfcAxis2Placement3D and IfcAxis2Placement2D with the right-handed axis derivation, IfcCartesianPoint, IfcDirection, IfcGridPlacement and IfcVirtualGridIntersection, and the IFC2x3 / IFC4 / IFC4.3 version differences. Keywords: IFC placement, IfcObjectPlacement, IfcLocalPlacement, PlacementRelTo, RelativePlacement, IfcAxis2Placement3D, IfcAxis2Placement2D, IfcCartesianPoint, IfcDirection, IfcGridPlacement, IfcVirtualGridIntersection, IfcLinearPlacement, placement tree, local coordinate system, world coordinates, transformation chain, element in wrong position, element at origin, model offset, why is my wall in the wrong place, how do I position an element, 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 : Geometry Placement
IFC stores an element's shape and its position separately. The geometry items
inside a representation are defined in a local coordinate system whose origin is
(0,0,0). Where that local system sits in the world is carried by
IfcProduct.ObjectPlacement, an IfcObjectPlacement.
This skill covers the placement tree : how placements chain parent to child, how an
axis placement defines a coordinate system, and how a consumer resolves a product to
world coordinates. Georeferencing (IfcMapConversion, IfcProjectedCRS, the
IfcSite reference latitude/longitude) is a separate concern : see
ifc-syntax-georeferencing.
Applies to IFC2x3, IFC4, and IFC4.3. One attribute moved between versions and IFC4.3
added a third placement subtype : see the Version Differences section.
Quick Reference
IfcObjectPlacement : the placement supertype
Verified verbatim (IFC4.3) :
ENTITY IfcObjectPlacement
ABSTRACT SUPERTYPE OF (ONEOF
(IfcGridPlacement
,IfcLinearPlacement
,IfcLocalPlacement));
PlacementRelTo : OPTIONAL IfcObjectPlacement;
INVERSE
PlacesObject : SET [0:?] OF IfcProduct FOR ObjectPlacement;
ReferencedByPlacements : SET [0:?] OF IfcObjectPlacement FOR PlacementRelTo;
END_ENTITY;
IfcObjectPlacement is abstract : NEVER instantiate it directly. Use a
concrete subtype.PlacementRelTo is the link to the parent placement. When omitted, the placement
is absolute in the project world coordinate system.PlacesObject (inverse) gives the products positioned by this placement.- In IFC4.3
PlacementRelTo is on IfcObjectPlacement, so every placement type
chains. In IFC2x3 and IFC4 PlacementRelTo was on IfcLocalPlacement only.
IfcLinearPlacement is an IFC4.3 addition : IFC2x3 and IFC4 have only
IfcGridPlacement and IfcLocalPlacement.
IfcLocalPlacement : the common placement
Verified verbatim (IFC4.3) :
ENTITY IfcLocalPlacement
SUBTYPE OF (IfcObjectPlacement);
RelativePlacement : IfcAxis2Placement;
WHERE
WR21 : IfcCorrectLocalPlacement(RelativePlacement, PlacementRelTo);
END_ENTITY;
RelativePlacement (IfcAxis2Placement, required) is the transformation from this
element's local system into the parent's system.PlacementRelTo is inherited (IFC4.3) or local (IFC2x3 / IFC4). When present it
names the parent placement ; when absent the placement is absolute.WR21 (IfcCorrectLocalPlacement) enforces dimensional consistency : a 3D
RelativePlacement may only chain to a 3D parent placement.
IfcAxis2Placement is a SELECT over IfcAxis2Placement2D and IfcAxis2Placement3D
(and IfcAxis2PlacementLinear in IFC4.3).
The placement tree
PlacementRelTo chains placements into a tree that mirrors the spatial structure :
IfcProject (world coordinate system, the root)
^
IfcSite placement PlacementRelTo -> none (or the project context)
^
IfcBuilding placement PlacementRelTo -> site placement
^
IfcBuildingStorey placement PlacementRelTo -> building placement
^
IfcWall placement PlacementRelTo -> storey placement
^
IfcDoor placement PlacementRelTo -> wall placement
Each level's RelativePlacement is the transform into its parent. ALWAYS set a
child element's PlacementRelTo to its spatial-structure parent's placement. NEVER
leave every element absolute : a model where each element is independently absolute
cannot be moved as a unit and breaks when the storey is repositioned.
Resolving to world coordinates
A product's world transform is the product of every RelativePlacement matrix
from the element up the PlacementRelTo chain to the root :
M_world(door) = M(site) x M(building) x M(storey) x M(wall) x M(door)
ALWAYS multiply the entire chain to obtain world coordinates. NEVER read a single
RelativePlacement as a world position : a RelativePlacement is relative to the
parent, not to the world, unless PlacementRelTo is absent.
IfcAxis2Placement3D : a 3D coordinate system
Verified verbatim (IFC4.3) :
ENTITY IfcAxis2Placement3D
SUBTYPE OF (IfcPlacement);
Axis : OPTIONAL IfcDirection;
RefDirection : OPTIONAL IfcDirection;
DERIVE
P : LIST [3:3] OF IfcDirection := IfcBuildAxes(Axis, RefDirection);
WHERE
AxisAndRefDirProvision : NOT ((EXISTS (Axis)) XOR (EXISTS (RefDirection)));
AxisIs3D : (NOT (EXISTS (Axis))) OR (Axis.Dim = 3);
AxisToRefDirPosition : (NOT (EXISTS (Axis))) OR (NOT (EXISTS (RefDirection)))
OR (IfcCrossProduct(Axis,RefDirection).Magnitude > 0.0);
LocationIs3D : SELF\IfcPlacement.Location.Dim = 3;
RefDirIs3D : (NOT (EXISTS (RefDirection))) OR (RefDirection.Dim = 3);
END_ENTITY;
Location is inherited from IfcPlacement : the origin, an IfcCartesianPoint.Axis is the Z axis (IfcDirection). Default when omitted : [0,0,1].RefDirection approximates the X axis. Default when omitted : [1,0,0].- The Y axis is derived by
IfcBuildAxes as the cross product, and RefDirection
is re-orthogonalised against Axis to yield a right-handed system.
AxisAndRefDirProvision : Axis and RefDirection MUST be both present or both
omitted. Supplying exactly one fails the rule.AxisToRefDirPosition : Axis and RefDirection MUST NOT be parallel : a zero
cross-product magnitude cannot form a coordinate system.
IfcAxis2Placement2D : a 2D coordinate system
Verified verbatim (IFC4.3) :
ENTITY IfcAxis2Placement2D
SUBTYPE OF (IfcPlacement);
RefDirection : OPTIONAL IfcDirection;
DERIVE
P : LIST [2:2] OF IfcDirection := IfcBuild2Axes(RefDirection);
WHERE
LocationIs2D : SELF\IfcPlacement.Location.Dim = 2;
RefDirIs2D : (NOT (EXISTS (RefDirection))) OR (RefDirection.Dim = 2);
END_ENTITY;
It has no Axis : a 2D system has only Location and RefDirection (the X axis,
default [1,0]). The Y axis is derived perpendicular to it.
IfcCartesianPoint and IfcDirection
ENTITY IfcCartesianPoint
SUBTYPE OF (IfcPoint);
Coordinates : LIST [1:3] OF IfcLengthMeasure;
WHERE
CP2Dor3D : HIINDEX(Coordinates) >= 2;
END_ENTITY;
ENTITY IfcDirection
SUBTYPE OF (IfcGeometricRepresentationItem);
DirectionRatios : LIST [2:3] OF IfcReal;
DERIVE
Dim : IfcDimensionCount := HIINDEX(DirectionRatios);
WHERE
MagnitudeGreaterZero : SIZEOF(QUERY(Tmp <* DirectionRatios | Tmp <> 0.0)) > 0;
END_ENTITY;
IfcCartesianPoint.Coordinates holds 2 or 3 length values (CP2Dor3D forbids a
1D point in practice).IfcDirection.DirectionRatios holds 2 or 3 ratios. A direction MUST have a
non-zero magnitude : an all-zero direction fails MagnitudeGreaterZero.IfcDirection is a direction, NOT a point : it carries no unit and is treated as
normalised.
IfcGridPlacement : placement on a structural grid
Verified verbatim (IFC4.3) :
ENTITY IfcGridPlacement
SUBTYPE OF (IfcObjectPlacement);
PlacementLocation : IfcVirtualGridIntersection;
PlacementRefDirection : OPTIONAL IfcGridPlacementDirectionSelect;
END_ENTITY;
ENTITY IfcVirtualGridIntersection;
IntersectingAxes : LIST [2:2] OF UNIQUE IfcGridAxis;
OffsetDistances : LIST [2:3] OF IfcLengthMeasure;
END_ENTITY;
PlacementLocation is an IfcVirtualGridIntersection : the meeting point of
exactly two IfcGridAxis instances, with OffsetDistances shifting the point
off the exact intersection.PlacementRefDirection (IfcGridPlacementDirectionSelect : an IfcDirection or an
IfcVirtualGridIntersection) orients the X axis. When omitted the grid axis
tangent is used.IfcGridPlacement positions an object on the design grid. See IfcGrid and
IfcGridAxis in ifc-syntax-geometry-representations.
Version differences (compact)
| Item | IFC2x3 | IFC4 | IFC4.3 |
|---|
PlacementRelTo declared on | IfcLocalPlacement | IfcLocalPlacement | IfcObjectPlacement |
IfcObjectPlacement subtypes | Grid, Local | Grid, Local | Grid, Linear, Local |
IfcLinearPlacement | absent | absent | present |
IfcAxis2PlacementLinear | absent | absent | present |
The placement-tree mechanics, IfcLocalPlacement, the axis placements,
IfcCartesianPoint, IfcDirection, and IfcGridPlacement are identical in syntax
across all three versions ; only the location of PlacementRelTo and the extra
IFC4.3 linear subtype differ.
Decision Trees
Which placement subtype?
How is the object positioned?
|
+-- Relative to another element or spatial container (the normal case)
| --> IfcLocalPlacement, PlacementRelTo set to the parent's placement.
|
+-- On a structural design grid, at a grid-axis intersection
| --> IfcGridPlacement with an IfcVirtualGridIntersection.
|
+-- Along an alignment / linear referencing curve (infrastructure) [IFC4.3 only]
--> IfcLinearPlacement. See ifc-syntax-georeferencing.
Absolute or relative placement?
Does this object sit inside a spatial container or host element?
|
+-- YES (a wall in a storey, a door in a wall) : ALWAYS relative.
| --> set PlacementRelTo to the parent's IfcObjectPlacement.
|
+-- NO, it is the spatial root (IfcSite at project origin)
--> PlacementRelTo omitted : the placement is absolute in the world system.
Axis placement : defaults or explicit axes?
Is the local system aligned with the parent axes (no rotation)?
|
+-- YES --> omit BOTH Axis and RefDirection. Defaults [0,0,1] and [1,0,0] apply.
|
+-- NO --> supply BOTH Axis and RefDirection. Supplying one only fails
the AxisAndRefDirProvision rule.
Patterns
Pattern : Build a relative placement chain
ALWAYS give each element an IfcLocalPlacement whose PlacementRelTo points at its
spatial-structure parent's placement : door to wall, wall to storey, storey to
building, building to site.
ALWAYS make the RelativePlacement express the offset and rotation from the parent
system, not from the world.
NEVER create a cycle : an element's PlacementRelTo chain MUST terminate at an
absolute (parentless) placement. A cycle makes world resolution non-terminating.
Pattern : Define an axis placement
ALWAYS use an IfcAxis2Placement3D for a 3D RelativePlacement and an
IfcAxis2Placement2D for a 2D one ; WR21 rejects a dimension mismatch with the
parent.
ALWAYS omit both Axis and RefDirection when the local system is axis-aligned with
the parent : the defaults [0,0,1] and [1,0,0] apply and the file stays compact.
NEVER supply only one of Axis and RefDirection : AxisAndRefDirProvision requires
both or neither.
NEVER set Axis and RefDirection parallel : AxisToRefDirPosition requires a
non-zero cross product.
Pattern : Place an element on a grid
ALWAYS model a grid-positioned element with an IfcGridPlacement whose
PlacementLocation is an IfcVirtualGridIntersection referencing exactly two
IfcGridAxis instances.
ALWAYS use OffsetDistances to shift the element off the exact axis intersection
rather than inventing a separate point.
NEVER reference the same IfcGridAxis twice in IntersectingAxes : the list is
UNIQUE and two axes are required to define an intersection.
Pattern : Resolve a product to world coordinates
ALWAYS resolve a world position by composing every RelativePlacement transform from
the element up the PlacementRelTo chain to the parentless root.
ALWAYS treat a placement with no PlacementRelTo as absolute : it is already in the
world coordinate system.
NEVER assume an element is at the world origin because its RelativePlacement origin
is (0,0,0) : (0,0,0) is relative to the parent, and the parent chain supplies the
real offset.
Pattern : Honour the version differences
ALWAYS read PlacementRelTo from IfcLocalPlacement itself in IFC2x3 and IFC4, and
from the IfcObjectPlacement supertype in IFC4.3 : the attribute moved up the
hierarchy, but it exists in every version.
NEVER write IfcLinearPlacement or IfcAxis2PlacementLinear in an IFC2x3 or IFC4
file : both are IFC4.3 additions and do not exist in the earlier schemas.
Reference Links
references/methods.md : the complete verified entity definitions for
IfcObjectPlacement, IfcLocalPlacement, IfcPlacement, IfcAxis2Placement3D,
IfcAxis2Placement2D, IfcCartesianPoint, IfcDirection, IfcGridPlacement,
IfcVirtualGridIntersection, the IfcAxis2Placement and
IfcGridPlacementDirectionSelect selects, and the per-version difference matrix.references/examples.md : worked STEP scenarios : an absolute site placement, a
four-level placement chain, a rotated axis placement, a grid placement, and reading
a product to world coordinates.references/anti-patterns.md : the placement failure modes this skill prevents,
each with the reason it fails.
Official sources
Related skills
ifc-syntax-geometry-representations : IfcShapeRepresentation, IfcGrid, and the
representation context whose WorldCoordinateSystem defines the project origin.ifc-syntax-georeferencing : IfcMapConversion, IfcProjectedCRS, the IfcSite
reference latitude/longitude, and IFC4.3 IfcLinearPlacement.ifc-impl-geometry-extraction : the workflow for reading geometry and resolving
world coordinates in practice.
