If you already do this for a living: Sign conventions. Two tools can show the same model rotated in opposite directions; write down the angle, the direction and which north, once, on one sheet.
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The architect models around the project base point, the engineer around the survey point, and the MEP consultant uses internal origin. The first federation shows three buildings stacked and turned. Two weeks go into re-exporting, and nobody is sure which one is right.
At kick-off the surveyor issues one georeferencing sheet: CRS and EPSG code, base point coordinates, rotation with its sign, and five control points. Every team sets up from it, and the first federation lines up within millimetres.
Federation goes wrong in a very visible way: the structural model floats ten metres above the architecture, or the services are rotated by a few degrees. The cause is almost always the same — teams did not agree where zero is.
Two problems, one answer
There are two separate questions:
- How do our models relate to each other? Everyone must use the same origin and orientation, often called shared coordinates.
- How do our models relate to the real world? That is georeferencing: tying the shared origin to a coordinate reference system (CRS) such as a national grid.
Answer both at the start of the project and write the answers into the BEP, ideally on a single sheet.
What the sheet records
- The CRS and its EPSG code (for example a national grid or a UTM zone), plus the vertical datum for heights.
- The project base point and survey point, with their grid coordinates.
- The rotation between the model's north and grid north.
- A set of control points measured by the surveyor, with their model coordinates.
Rotation, translation and scale
Mapping model coordinates to grid coordinates needs a translation (where the model origin sits on the grid), a rotation (how the model is turned) and sometimes a scale (usually 1 for buildings). IFC4 stores exactly these values in an entity called IfcMapConversion, linked to the CRS.
Be careful with north: grid north and true north differ by a small angle that depends on where the site sits in the projection. Authoring tools also use different sign conventions for rotation. Always say which one you mean.
For example grid north entered instead of true north, or the sign of the angle flipped.
Half a degree sounds like nothing. 100 m from the origin it moves the model by 873 mm, enough to put a riser outside its shaft. That is why rotation is written down once, with its sign convention, and checked with control points.
- 1Survey controlGrid E, N, H from the surveyor
- 2Same points in the modelModel X, Y, Z
- 4Residual checkMillimetres, not metres
- 5IfcMapConversionInto the IFC and the BEP
Check with control points
Two control points define the transformation exactly, so they cannot reveal a mistake. With three or more, compute the best fit and look at the residuals — the differences between where the transformation puts each point and where it was surveyed. Residuals of a few millimetres mean the model is placed correctly. A large residual at one point usually means a typo or a mislabelled point.
Key takeaways
- Every model has an internal origin; teams must agree how their models relate to each other and to the site.
- Georeferencing links the model to a projected coordinate reference system, identified by an EPSG code.
- A rotation and a translation (and sometimes a scale) describe how model coordinates map to grid coordinates.
- Check the result with surveyed control points: small residuals mean the model is where it should be.
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Related tool
Coordinate system, base points and survey control, with a best-fit check and IfcMapConversion values.
Open Georeferencing Sheet →