coordinates

Universal Transverse Mercator Coordinates

In the early twentieth century, The UK Ordnance Survey used longitude and latitude to define positions as seen on this 1938 map of Guildford . If you look at the corners of the map and zoom in, you'll see that it shows longitude along the top and bottom and latitude along each side.

In the 1940s the OS and some other national mapping authorities adopted Universal Transverse Mercator (UTM) coordinates. This approach divides the Ellipsoid into 60 zones, each covering six degrees of longitude. The local surface of each zone is projected onto a flat map and each position on the surface of the Ellipsoid is represented by its distance along an east-west line (its easting) from a zero point and its distance along a north-south line (its northing) from the same point. More details on UTM are here.

A position in three dimensions can be represented by its easting, northing and its height above some base, such as local mean sea level.

During its 1925 to 1962 re-survey of the UK, The Ordnance Survey created the National Grid reference system (OSGB, strictly OSGB36), also known as British National Grid (BNG). It's based on the Airy Spheroid, produced in the 19th Century. More about OSGB36 here .

The zero point for OSGB eastings and northings is out at sea to the south-west of Land's End at latitude 49.7668065515° longitude -7.5572420349°.

The notional Mean Sea Level was fixed in the 1920's using the facility at Newlyn. It's about 55 metres above the Ellipsoid.

OS maps printed since the re-survey have shown eastings and northings around the edges. Contour lines show height above sea level.

By the time of the survey the UK scientific community has adopted International Systems (metric) units, but the rest of the country still used the old Imperial units - inches, feet and miles. The OS teams did their measurements in metres but continued to draw their maps in Imperial units, using scales such as six inches to the mile. When the UK went metric in 1971, they rescaled the maps to metric units.

OSGB is defined in terms of reference points on the ground laid out during the survey. The OS produced a network of physical markers called Trig Points. The most recognisable of these are four-foot high concrete pillars built at strategic points around the country, usually on high ground so that they can be seen from a distance. These trig points move with continental drift so the OSGB36 grid moves with them.

Modern survey equipment is accurate enough to reveal flaws in the Airy Spheroid, making conversion from GPS coordinates to OSGB36 difficult and error-prone.

The UK Ordnance Survey provide this web page which converts between the various formats. It can take a position expressed as latitude, longitude and height above the Ellipsoid in the ETRF89 reference frame and convert it to ECEF coordinates and OSGB36 easting, northings and height above mean sea level, but it seems to introduce a significant error. That's a nuisance because here in the UK we've been creating records of positions using OSGB36 for the last sixty years

So I have GNSS equipment that can find my position within a couple of centimetres, but if I use that OS web page to convert it to an OS grid reference, I lose a lot of that accuracy.

The OS also provide a spreadsheet that you can download and run on your own computer to convert coordinates from one form to another. I don't know if that's any better.