National grid converter

The systems and how accurate each one is

Getting from latitude and longitude to a national grid takes two steps. The projection (the math that turns degrees into meters on a flat grid) is exact to the millimeter for every system here. The datum change is where the error comes in: some grids sit on ETRS89, which matches GPS to well under a meter, and others sit on older national datums that need a shift.

“Exact” means exact with respect to ETRS89. GPS and Google Maps work in WGS84, which in Europe today differs from ETRS89 by a little under 1 m. That only matters for survey work.

US State Plane coordinates

Each US state has its own grid, the State Plane Coordinate System of 1983 (SPCS83), on the NAD83 datum: one to ten zones per state, defined county by county, each on a Transverse Mercator or a Lambert projection (an oblique Mercator in the Alaska panhandle) so that distances are off by no more than 1 part in 10,000. Choose “State Plane (NAD83)” and the converter finds each point’s zone from the county it falls in, or pick the zone yourself. Results come in meters or in the feet each state uses: US survey feet in most, international feet in Arizona, Michigan, Montana, North Dakota, Oregon and South Carolina, and either in Utah. Kentucky gets its single statewide zone, as in the NGS’s own tools, with the North or South zone as an alternative.

The projection matches PROJ to a tenth of a millimeter in all 124 zones, and NCAT, the National Geodetic Survey’s converter, to the millimeter. The datum is where the error comes in: like PROJ by default, the converter treats NAD83 as WGS84, and today the two are 1 to 2 m (3 to 7 ft) apart in the lower 48. That’s fine for mapping and GIS; for survey work, use NGS tools with a proper NAD83(2011) transformation.

When you need more than this

For the British National Grid, the converter uses OSTN15, the transformation that defines the National Grid, in the NTv2 version Ordnance Survey publishes, the same one PROJ and QGIS use: it lands within 7 mm of the 40 official OS test points. For the Netherlands, it follows NSGI’s official procedure with its rdcorr2018 correction grid and matches all 9,758 of NSGI’s test points to the millimeter (positions only: NAP heights aren’t converted). For mapping, navigation, hiking, real estate or checking where a point is, that’s plenty; for setting out a building or a boundary, use a surveyor.

Germany and Spain work the same way: the converter uses the official national grid files (BeTA2007 and the Spanish IGN’s SPED2ETV2), the same ones PROJ, QGIS and ArcGIS use, interpolated in your browser. Grid files are only downloaded when you pick a system that needs them, and the British one comes in 18 pieces of about 25 KB, so only the ones around your points are fetched.

Tips for each country

• United Kingdom. You can paste OS grid references with letters (“TQ 302 796” or “TQ 30268 79643”) as well as plain eastings and northings, and the results show both.
• Ireland. Today’s grid is ITM (eastings around 400 000 to 800 000). The older Irish Grid, still printed on Northern Ireland’s OSNI maps and on older Irish ones, uses a single letter per 100 km square (“O 158 346” is central Dublin); paste it with or without the letter. The same point is about 400 km further east and 500 km further north in ITM than in the Irish Grid, so it’s easy to tell which one you have.
• Germany. The first digit of a Gauss-Krüger easting (Rechtswert) is the zone: 4 593 627 is zone 4, centered on 12° E. Today’s official system is ETRS89/UTM zones 32 and 33, which you get with “UTM (WGS84).”
• Switzerland. LV95 eastings start with 2 and northings with 1 (2 600 000 / 1 200 000 is the old observatory in Bern). The older LV03 drops those leading digits: adding 2 000 000 and 1 000 000 gets you close, but not exactly, because the official conversion also corrects distortions in the old network.
• France. Lambert-93 eastings are around 100 000 to 1 200 000 and northings 6 000 000 to 7 200 000. The old Lambert II étendu (NTF) isn’t supported.
• Spain. Use the zone your data was produced in, even if the point falls in the next one; ED50 coordinates are about 230 m away from today’s.