Knowledge base
Coordinate reference systems and height datums
A model is only usable when it is in the same system as the machine. The coordinate systems and height datums commonly used in each country, what a local site calibration does and what happens when the wrong system is chosen.
Two references: position and height
Every coordinate in a model consists of a position (X and Y, or easting and northing) and a height (Z). Each needs a reference. The position is expressed in a coordinate reference system with its own projection; the height is measured against a national height datum, usually tied to a mean sea level.
GNSS receivers natively measure in a global frame (ETRS89 in Europe) with an ellipsoidal height. The machine control system converts those measurements to the national grid and, through a geoid model, to the national height datum. The model must be in exactly that same system.
Common systems by country
Below are the systems most often found on drawings. A project may always be drawn in another or a local system, which is why we always check.
- Belgium: Belgian Lambert 72 or Lambert 2008, height TAW/DNG (Ostend).
- Netherlands: RD New (Amersfoort), height NAP.
- France: Lambert-93 or one of the conic zones CC42 to CC50, height NGF-IGN69.
- Germany: ETRS89 / UTM (zone 32 or 33), height DHHN2016.
- Luxembourg: LUREF (Luxembourg TM), height NG95.
- United Kingdom: OSGB36 / British National Grid, height ODN (Newlyn).
- Switzerland: CH1903+ / LV95, height LN02.
- Other European countries: often ETRS89 / UTM in the correct zone, with EVRF2007 or a national height system.
Local site calibration
On many sites work is not done directly in the national grid but with a local site calibration (also called localisation). A few known control points on site are observed, and a small shift, rotation and possibly scale are computed so that the GNSS measurements fit those points exactly.
A calibration compensates for inaccuracies in older drawings or in transformations between systems, but it is only valid within the area enclosed by the points used. Outside that area, deviations can grow quickly. Setting up and maintaining the calibration on the machine remains the responsibility of the contractor or their surveyor; we make sure the model is in the agreed system.
What happens with the wrong system
A wrong system rarely produces an error message. The model loads normally but sits in the wrong place or at the wrong height. The consequences vary:
- A completely different system places the model kilometres away or even in another country; the machine cannot find it.
- Two related systems (for example two versions of a national grid) cause a shift of a few centimetres up to several hundred metres.
- A wrong height datum or missing geoid model causes a height error from centimetres to more than a metre, which does not stand out on the display.
- Hence the rule: always check on a known point before starting, both in position and in height.