Georeferencing and the location map
Summary
A model that knows where it is can be combined with maps, surveys, terrain, neighbouring models and the owner's asset register. A model that doesn't is a drawing floating at the origin of its own coordinate system, and someone has to place it by hand every time it is used. The Georeferencing check on the Validity page tells you which of the two you received, and the location map shows the result.
IfcMapConversion (where the model's origin sits, and how it is rotated and scaled) together with an IfcProjectedCRS (which grid, usually by its EPSG code). This exists in IFC4 and IFC 4.3. IFC2X3 can only store the latitude and longitude of the site.
The three levels
| Level | What the file contains | Score |
|---|---|---|
| Full CRS | IfcMapConversion with a projected CRS. The report tags it LoGeoRef 50. | 100 |
| Lat/long | Only the site's reference latitude and longitude (IfcSite.RefLatitude and RefLongitude), not both zero. The report tags it LoGeoRef 20. | 50, or 75 for IFC2X3, where this is the most the schema can hold |
| None | No usable location. | 0 |
A latitude and longitude place the site to within metres at best and say nothing about rotation, so they are not enough to line a model up with a survey. For every file that is not a building, such as roads, railways and bridges, the check weighs one and a half times as much, because those are almost always combined with other geographic data.
Problems we look for
Having the right entities is not the same as having the right location. The check lowers the score, or sets it to zero, for these:
- Default location. The coordinates match a known template location of an authoring tool, or a well-known origin such as 0° N, 0° E or the origin of the Dutch RD grid. The site was never set up. The check scores 0 and the Validity page shows Default Software Geolocation Detected.
- Default map conversion. Some exporters write a map conversion of 0, 0, 0 in Web Mercator (EPSG:3857) when nothing was configured. It is treated like a default location: the check scores 0.
- Zero origin with a real CRS. The map conversion says the model origin is at 0, 0, 0 of a real grid, while the geometry itself is drawn at large world coordinates. It works, but many tools lose precision far from the origin. Set the project base point to the real map coordinates and model near the local origin instead. (−15)
- Outside the CRS's area of use. Every projected CRS is defined for a region. When the model's eastings and northings fall outside that region, or cannot be converted at all, the wrong CRS was almost certainly chosen. (−20)
- Default address. The site address still contains the template text of the authoring tool. (−15)
Only one of the deductions applies at a time. The check's value in the report names the problem, and its advice says what to change.
The location map
When the file has eastings, northings and a CRS, the Validity page draws the model on a map at its real position. The map's coordinates are calculated from the file's own map conversion: rotation, scale and units included, then converted to latitude and longitude. If that is not possible, the map falls back to the site's latitude and longitude.
- Basemaps: streets (the default), OpenStreetMap, satellite and topographic.
- Layers: the footprint of the model as measured from its geometry, the alignments of roads and railways, and the map conversion origin. Click the origin to see its eastings, northings, height, CRS and latitude/longitude.
- 2D and 3D: 2D draws the footprint, alignments and origin from the report. 3D loads the model itself, only when you switch to it. Snap to ground drops a model that floats high above the map to ground level; X-ray draws the model through the terrain and buildings around it.
- Zoom to model brings the model back into view.
Look at the map with a critical eye. A model in the sea, in the wrong country, rotated against the street pattern or a factor of a thousand too large is a georeferencing error that no score fully captures. A map that cannot be drawn because WebGL is switched off in the browser shows a link to how to fix that.
The map needs the live platform, so the standalone HTML download shows the coordinates and the georeferencing facts instead. On a shared secret link the 3D model is not available; only the owner of the data can load it.
The Georeferenced capability
A file earns the Georeferenced capability when it has full georeferencing (a map conversion with a projected CRS) and no default location. IFC2X3 files cannot earn it, because the schema has no map conversion.
Getting it right
- Export to IFC4 or IFC 4.3 when location matters. It is the only way to store full georeferencing.
- Set the survey point and project base point in the authoring tool to the real coordinates of the site, and choose the projected CRS the project uses (for example a national grid or a UTM zone, by its EPSG code).
- Model near the local origin and let the map conversion carry the offset to world coordinates.
- Fill in the site address instead of leaving the template text.
- Check the result on the location map: position, rotation and size.
Frequently asked questions
My file has latitude and longitude. Why is that only half the score?
Because it only tells roughly where the site is. Without a projected CRS and a map conversion, the model cannot be aligned with surveys or other models to the centimetre, and its rotation is unknown. In IFC2X3, where nothing better is possible, it scores 75.
The map shows my model in the wrong place. What went wrong?
Usually one of three things: the wrong CRS (the check then often reports outside the CRS area of use), eastings and northings swapped or in the wrong unit, or the project base point not set in the authoring tool. Click the origin on the map to see what the file says.
Why is there no map for my file?
The map needs eastings, northings and a CRS in the file. Files with only a latitude and longitude, or no location at all, get the facts table without a map.