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Section 7 - Linear Placement

Many elements in the built environment for infrastructure works are located relative to an alignment. The IFC class IfcLinearPlacement accomplishes this type of object placement.

Generally alignments are place in the project coordinate system so they are relative to (0,0). To indicate that a linear placement is relative to the alignment, the PlacementRelTo attribute is omitted indicating that the placement is relative to the start of the basis curve of the alignment.

The IfcLinearPlacement.RelativePlacement attribute places an object. RelativePlacement is an IfcAxis2PlacementLinear.

IfcAxis2PlacementLinear.Location is an IfcPoint, but is constrained to be an IfcPointByDistanceExpression by a where rule. This means that the placement location is measured along the basis curve. For a 3D alignment curve such as IfcGradientCurve or IfcSegmentedReferenceCurve, the basis curve is IfcCompositeCurve representing the projection of the 3D curve onto a 2D horizontal plane.

General

IfcAxis2PlacementLinear has optional Axis and RefDirection attributes. When omitted, RefDirection is the tangent to the 3D curve at Location.

Unfortunately, Axis is not clearly defined in the IFC schema documentation (See Default value for IfcAxis2PlacementLinear.Axis is not defined · Issue #125 · buildingSMART/IFC4.x-IF).

The most logical default Axis is "perpendicular" to the RefDirection in the general direction of global Z-axis. However, the most common interpetation is that Axis is up (0,0,1).

When Axis and RefDirection are not provided, the local coordinate system can be computed as follows:

  1. Set Axis = Z = (0,0,1)
  2. Compute the tangent vector to the 3D curve. This is the RefDirection.
  3. Y = cross product of Z and RefDirection.
  4. X = cross product of Y and Z.

X, Y, Z are orthoginal axes.

Linear Placement along IfcOffsetCurveByDistances

Linear placement along an IfcOffsetCurveByDistances is not precise. IfcOffsetCurveByDistances is an interpoloated curve defined by offsets from a basis curve. The offset values are linearly interpolated forming a segmented curve. The length of the offset curve is approximate and depends on the frequency of the interpoloated points along the curve. For this reason, IfcAxis2PlacementLinear.Location.DistanceAlong cannot be precisely determined.