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Derive map units from IFC scale

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Use IfcMapConversion.Scale as the source of truth for converting map coordinates to metres, instead of deriving that scale from IfcProjectedCRS.MapUnit. Bump the sidecar version because cached georef matrices and unit scales may differ under the new interpretation.

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This commit is contained in:
Dion Moult
2026-05-19 18:58:17 +10:00
parent e0a504417c
commit 6ca38f8bf6
7 changed files with 41 additions and 24 deletions
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src/ifcviewer/Geolocation.h
+8 -5
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@@ -81,9 +81,11 @@ Eigen::Matrix4d autoLocal2Global(ifcopenshell::file* ifc_file,
// [ R_z(theta) · diag(fx, fy, fz) | (e, n, h) · u_m ]
// [ 0 | 1 ]
//
// `map_unit_to_meters` is the SI scale of IfcProjectedCRS.MapUnit (or the
// project's LENGTHUNIT scale if MapUnit is absent). The caller composes any
// IfcGeometricRepresentationContext WCS on the right:
// `map_unit_to_meters` is derived by the caller from the IFC project length
// unit and the authoritative IfcMapConversion.Scale. Since this matrix takes
// meter inputs from the geometry iterator, Scale is represented by that unit
// conversion and is not applied again in the linear block. The caller composes
// any IfcGeometricRepresentationContext WCS on the right:
// G = helmertMetersFromParameters(...) · inv(wcs_meters)
// (where wcs_meters has its translation column converted from project units
// to meters via calculateUnitScale).
@@ -96,8 +98,9 @@ Eigen::Matrix4d helmertMetersFromParameters(const HelmertTransformation& params,
// IfcCoordinateOperation.TargetCRS.MapUnit (the IfcNamedUnit), if present.
// Returns nullopt for IFC2X3, models without an IfcCoordinateOperation, or
// when MapUnit is absent on the IfcProjectedCRS. Callers fall back to
// calculateUnitScale(file, "LENGTHUNIT") in that case.
// when MapUnit is absent on the IfcProjectedCRS. This is retained for UI /
// metadata inspection; transform composition derives map unit scale from
// IfcMapConversion.Scale instead.
std::optional<express::Base> getMapUnit(ifcopenshell::file* ifc_file);
// "How do I rotate project east to get to grid east?" — i.e. -atan2(xao, xaa)