Default value for IfcAxis2PlacementLinear.Axis is not defined

[RickBrice]

When trying to compute placements for comparison with https://github.com/buildingSMART/IFC4.x-IF/tree/main/IFC-files/Linear-placement-sleepers/ACCA#readme the following issue is noted.

Problem
The default value of IfcAxis2PlacementLinear.Axis is not defined. Using IfcAxis2Placement3D for inspiration, you could say that the default Axis is (0,0,1). However, IfcAxis2PlacementLinear.RefDirection, when omitted, is taken from the curve tangent at IfcAxis2PlacementLinear.Location so you could infer that Axis should be derived from the IfcPointByDistanceExpression.BasisCurve as the direction perpendicular to the curve in the "Distance Along - Elevation" plane.

See further discussion and illustrations in buildingSMART/IFC4.3.x-development#732

Additionally, the question as to whether both or neither IfcAxis2PlacementLinear.Axis and RefDirection are needed similar to IfcAxis2Placement3D requirements. See buildingSMART/IFC4.3.x-development#731.

Solution(s)

• Clarify documentation regarding default values for Axis and RefDirection
• Provide clear semantics on permutations of existence of the two direction vectors (and possibly constrain, eg for 3d we have a formal prop "Either both, Axis and RefDirection are not given and therefore defaulted, or both shall be given.", see Are both or neither Axis and RefDirection required for IfcAxis2PlacementLinear IFC4.3.x-development#731)
• Provide derived attribute referencing IfcBuildAxes

Require schema changes?

• yes
• ✓ no
• don't know

Require documentation changes?

• ✓ yes
• no
• don't know

[RickBrice]

A solution has been proposed in buildingSMART/IFC4.3.x-development#732 (comment).

To summarize the proposal:

• Augment IfcAxis2PlacementLinear documentation to state clearly that orthogonalization is implemented opposite as it is for IfcAxis2Placement3D
• Provide clear semantics on permutations of existence of the two direction vectors (and possibly constrain, eg for 3d we have a formal prop "Either both, Axis and RefDirection are not given and therefore defaulted, or both shall be given.", edit: see Are both or neither Axis and RefDirection required for IfcAxis2PlacementLinear IFC4.3.x-development#731)
• Provide derived attribute referencing IfcBuildAxes as DERIVE P : LIST [3:3] OF IfcDirection := IfcBuildAxes(RefDirection,Axis) in Section 8.9.3.3.5

Is there sufficient agreement to draft an Implementation Agreement for https://standards.buildingsmart.org/documents/Implementation/IFC_Implementation_Agreements/.

I'm guessing implementation agreements are a very difficult thing to do since the last two listed are from 2014 and are still proposals in progress. If there is agreement on the path forward, I willing to help bring this proposal to a formal conclusion.