A coordinate reference system (CRS) defines what a pair or triple of coordinates means: which model of the Earth they refer to, where the origin is, which way the axes point and in which units they are measured. The same numbers mean different places in different systems, so a coordinate without its CRS is incomplete.
At a glance
| Parts | Datum (model of the Earth), coordinate system (axes and units), and for grids a map projection |
| Identified by | An EPSG code, for example EPSG:27700 (British National Grid) or EPSG:4326 (WGS 84 latitude/longitude) |
| Registry | The EPSG Geodetic Parameter Dataset, maintained by IOGP |
| In BIM | Recorded in the BEP and in the model (IFC 4 and later: IfcProjectedCRS and IfcMapConversion) |
Types of CRS
| Type | Coordinates | Example | Used for |
|---|---|---|---|
| Geographic | Latitude, longitude in degrees | WGS 84 (EPSG:4326) | GPS, global data |
| Projected | Easting, northing in metres | UTM zones, national grids | Survey, site plans, BIM on site |
| Vertical | Height above a reference surface | Ordnance Datum Newlyn, NAVD88 | Levels and heights |
| Compound | Horizontal plus vertical | EPSG:7405 (British National Grid + ODN height) | Full 3D positions |
Datums
A datum fixes a model of the Earth's shape (an ellipsoid) to the real Earth. Different datums put the same physical point at coordinates that can differ by metres. Converting between them needs a transformation, some of which are approximations with known accuracy.
Projections and distortion
A projected CRS flattens part of the curved Earth onto a plane. Flattening always distorts something. Two effects matter on projects:
- Scale factor: a distance on the grid differs slightly from the same distance on the ground. On long linear projects (roads, rail) the difference adds up.
- Grid convergence: grid north differs from true north by an angle that changes across the zone.
Engineering projects often use a local site grid derived from the national grid to keep these effects small.
Why EPSG codes matter
Names such as "OSGB" or "UTM" are ambiguous: there are several versions, zones and height references. An EPSG code names exactly one definition. Stating the horizontal and vertical codes in the BEP and the georeferencing sheet removes a whole class of positioning errors.
Common mistakes
- Mixing grid north, true north and project north.
- Sharing coordinates without saying whether they are grid or ground distances.
- Leaving out the vertical datum, so levels are relative to an unknown zero.
- Converting between datums with a default transformation without checking its accuracy.
Where to read more
The EPSG registry (epsg.org) lists every code with its definition. National mapping agencies publish guidance for their grids. The project survey brief and georeferencing sheet should state the systems used.