IFC (IFC4X3) development IfcSectionedSpine Semantic definition

An IfcSectionedSpine is a representation of the shape of a three dimensional object composed by a number of planar cross sections, and a spine curve. The shape is defined between the first element of cross sections and the last element of the cross sections. A sectioned spine may be used to represent a surface or a solid but the interpolation of the shape between the cross sections is not defined.

All cross sections have to define areas by a closed profile to allow for the representation of a solid. All cross sections have to define curves by an open or closed profile to allow for the representation of a surface. The cross sections are defined by subtypes of IfcProfileDef, where the consecutive profiles may be derived by a transformation of the start profile or the previous consecutive profile.

The spine curve shall be of type IfcCompositeCurve, each of its segments represented by IfcCompositeCurveSegment shall correspond to the part between exactly two consecutive cross-sections.

Figure 1 illustrates an example of an IfcSectionedSpine.

  • The SpineCurve is given by an IfcCompositeCurve with two Segments. The Segments[1] has a ParentCurve of type IfcPolyline and a Transition = CONTSAMEGRADIENT. The Segments[2] has a ParentCurve of type IfcTrimmedCurve and a Transition = DISCONTINUOUS.
  • Each CrossSectionPosition lies at a start or end point of the Segments.
  • Each CrossSections are inserted by the CrossSectionPositions. The first two cross sections are of type IfcRectangleProfileDef, the third is of type IfcDerivedProfileDef.
spine 1
Figure illustrates the final result of the IfcSectionedSpine. The body (shown transparently) is not fully defined by the exchange definition.
Figure — Sectioned spine result

Informal Propositions:

  1. none of the cross sections, after being placed by the cross section positions, shall intersect
  2. none of the cross sections, after being placed by the cross section positions, shall lie in the same plane
  3. the local origin of each cross section position shall lie at the beginning or end of a composite curve segment. Entity inheritance Attributes

# Attribute Type Description
IfcRepresentationItem (2)
LayerAssignment SET [0:1] OF IfcPresentationLayerAssignment FOR AssignedItems

Assignment of the representation item to a single or multiple layer(s). The LayerAssignments can override a LayerAssignments of the IfcRepresentation it is used within the list of Items.

StyledByItem SET [0:1] OF IfcStyledItem FOR Item

Reference to the IfcStyledItem that provides presentation information to the representation, e.g. a curve style, including colour and thickness to a geometric curve.

Click to show 2 hidden inherited attributes Click to hide 2 inherited attributes
IfcSectionedSpine (4)
1 SpineCurve IfcCompositeCurve

A single composite curve, that defines the spine curve. Each of the composite curve segments correspond to the part between two cross-sections.

2 CrossSections LIST [2:?] OF IfcProfileDef

A list of at least two cross sections, each defined within the xy plane of the position coordinate system of the cross section. The position coordinate system is given by the corresponding list CrossSectionPositions.

3 CrossSectionPositions LIST [2:?] OF IfcAxis2Placement3D

Position coordinate systems for the cross sections that form the sectioned spine. The profiles defining the cross sections are positioned within the xy plane of the corresponding position coordinate system.

* Dim IfcDimensionCount

This attribute is formally derived.


The dimensionality of the spine curve is always 3.

Table Formal propositions

Name Description

The profile type (either AREA or CURVE) shall be consistent within the list of the profiles defining the cross sections.

SIZEOF(QUERY(temp <* CrossSections | CrossSections[1].ProfileType <> temp.ProfileType)) = 0

The set of cross sections and the set of cross section positions shall be of the same size.

SIZEOF(CrossSections) = SIZEOF(CrossSectionPositions)

The curve entity which is the underlying spine curve shall have the dimensionality of 3.

SpineCurve.Dim = 3
Table Formal representation

ENTITY IfcSectionedSpine
 SUBTYPE OF (IfcGeometricRepresentationItem);
	SpineCurve : IfcCompositeCurve;
	CrossSections : LIST [2:?] OF IfcProfileDef;
	CrossSectionPositions : LIST [2:?] OF IfcAxis2Placement3D;
	 Dim : IfcDimensionCount := 3;
	ConsistentProfileTypes : SIZEOF(QUERY(temp <* CrossSections | CrossSections[1].ProfileType <> temp.ProfileType)) = 0;
	CorrespondingSectionPositions : SIZEOF(CrossSections) = SIZEOF(CrossSectionPositions);
	SpineCurveDim : SpineCurve.Dim = 3;
END_ENTITY; References

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  • where rule, ConsistentProfileTypes
  • where rule, CorrespondingSectionPositions
  • where rule, SpineCurveDim
  • where rule, WR1
  • where rule, WR2
  • where rule, WR3