Land surveying in the UK: OSGB36 and the National Grid
Everyone in Britain who drops a raw GPS coordinate onto the map meets the same surprise: the point is a hundred metres off.
Field takeaways
- The national system is the National Grid on the OSGB36 datum (EPSG 27700).
- Grid references start with a letter pair naming the 100 km square.
- The OSGB36-WGS84 gap varies by region and can exceed a hundred metres.
- Precise conversion uses the national transformation model; one fixed formula is not enough.
- The app opens in the UK with the official system covering the location.
The National Grid: a system that starts with letters
The UK's national system is the National Grid on the OSGB36 datum (EPSG 27700). Its most visible feature is the lettered reference: pairs like TQ, SU or NY name a 100-kilometre square, and the digits that follow locate the point within it. In fully numeric form, eastings and northings are written in metres.
The gap to WGS84
OSGB36 is an old datum, and its difference from the WGS84 a GPS natively produces varies by region — in places beyond a hundred metres. Dropping the phone's raw coordinate straight onto a National Grid map therefore does not work. Nor is the gap a fixed shift: it differs from one end of the country to the other.
The right way to convert
Precise work uses the national transformation model; single-parameter shortcuts leave metre-level residuals. Fortunately this is not something a user does by hand — a properly defined catalogue applies the model behind the scenes.
What the app does in the UK
MapLab Survey opens in the UK with the official system covering the location. National grids, UTM and OSGB36 sit together in the catalogue; you can measure with GNSS, view in the National Grid, and export the delivery as DXF or CSV in the requested system.
The practical check
At a known point — a trig pillar, a corner — compare the phone's measurement with the grid value. A metre-level difference means the transformation is working; a hundred-metre one means you are looking at raw WGS84.
Try these steps on your own phone.
MapLab Survey handles RTK/GNSS point capture, drawing, area and volume calculations and coordinate conversion, then exports to DXF, KML/KMZ, GeoJSON, Shapefile, GeoPackage, NCZ or CSV. Capture, calculation and export work offline. Live NTRIP corrections and online basemaps need a connection.
Frequently asked questions
Which system is official in the UK?
The National Grid on the OSGB36 datum (EPSG 27700).
What do letters like TQ and SU mean?
Each letter pair names a 100-kilometre square; the digits after it locate the point within that square.
Why is my GPS coordinate a hundred metres off on the map?
That is the datum gap between raw WGS84 and OSGB36; the national transformation model closes it.
Which system does the app open with in the UK?
The official system covering the location; OSGB36 and UTM sit together in the catalogue.
Technical references
The field guidance in this article is aligned with the technical and official references below.
Related: Coordinate conversion app · Placing a scanned sheet · Bluetooth RTK GNSS · MapLab Survey