Georeferencing links a model's own coordinate system to a real-world coordinate reference system (CRS), so the model sits in its true position, orientation and height. Building models are usually drawn near a local origin for precision; georeferencing records how that local system maps to the survey grid.
At a glance
| Inputs | The CRS (EPSG codes), a known point in both systems, the rotation to grid north, a scale |
| In IFC | IfcMapConversion and IfcProjectedCRS (IFC 4 and later); IfcSite latitude/longitude in IFC 2x3 |
| In authoring tools | Project base point, survey point and shared coordinates (names vary by software) |
| Recorded in | The BEP or a georeferencing sheet, agreed with the surveyor |
The four values
A 2D georeference needs:
| Value | Meaning |
|---|---|
| Eastings and northings | Where the model's origin lies on the grid |
| Orthogonal height | The height of the model's zero above the vertical datum |
| Rotation | The angle between the model's Y axis and grid north |
| Scale | Usually 1; differs when model units are converted or a grid scale factor is applied |
- 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
Why it matters
- Models from different teams line up without manual moves.
- Site models, surveys, utilities and GIS data can be combined.
- Setting out on site uses the same coordinates as the design.
- Infrastructure projects, which span kilometres, cannot work without it.
Checking a georeference
A georeference is checked against control points: points known by survey and modelled in the model. Each point gives a residual, the difference between where the model puts it and where the survey says it is. Two points always fit perfectly, so at least three are needed to reveal an error.
Common mistakes
- Moving the model to the survey coordinates instead of recording a transformation, which loses precision far from the origin.
- Confusing true north and grid north, often by the grid convergence angle.
- No vertical datum, or a height relative to finished floor instead of the datum.
- Each team georeferencing on its own, so the models disagree by a few millimetres that become centimetres on site.
- Exporting without the map conversion, so the IFC file arrives at the local origin.
Where to read more
buildingSMART's guidance on georeferencing in IFC explains IfcMapConversion. The surveyor's control report and the project's coordinate strategy give the actual values.