SLHS: Understanding British OS Grid Coordinates

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Hover Over The Map And See The Conflicts

I’m going to disappoint any Flat-Earthers by letting the secret out that the earth is curved -approximately spherical. Yet our tables and floors are flat. We like our maps to be too. If you hover your mouse over the UK map (left) you’ll see that there are two major conflicts.

1. 1.Firstly an atlas based coordinate system, being curved and sort of conical, cannot provide a map where the distance measured on the map is the same in different places. There can’t be a fixed scale which is vital for calculating distances -vital for a moving army and one that is shelling its enemy !
   

2. 2.Secondly London is on the right-hand side of the country and so using the Greenwich Meridian as the origin puts the whole map skew-whiff. They therefore centred on the 358º parallel.
   

This is a beautiful system of 100km x 100km squares. Stratford is under the “S” in square SP.

Refined To Perfection

That grid system is further refined to 10km x 10km squares and again down to 1km x 1km squares (see below).

When researching your favourite subject you may likely know where it is but if you don’t thankfully most local and county councils use a universal grid. This page is here to help us all understand the original mapping of places in the UK. I will also give a link to an excellent universal map location finder. Here’s a quick history lesson..

Navy Mapping

Famously in 1714 the British Admiralty offered £20,000 (equivalent to £3.3M in 2024) to anyone who could guarantee accurate navigation of His Majesty’s Navy from their zero point on a hill in London: the Greenwich Meridian. If you were in the Scouts or Guides you will have been taught that they used a compass and John Harrison’s famous clock to find their way about. From experience they mapped the coastlines and plumbed the depths of the seas. But what about the land ? Whilst a compass is helpful to know what direction you are facing it doesn’t give you an X and Y coordinate of where you are.

Army Mapping

The modern internationally agreed Global Positioning System (GPS) is based upon that magnetic system and ultimately a whole ream of satellites in geo-stationary orbit above the earth. When using a map app or taking a photograph all mobile phones record those coordinates.

We should be grateful for our wonderful map grid system to the rebellion of the Scots in 1745. The British Army needed to know where the Scots were hiding and so their Royal Ordinance department started the laborious task of surveying the whole of Scotland -no mean task. Later the rest of the UK was likewise mapped. As the cost of printing came down, and with computerisation, maps are cheaply available for many tasks. Since 2015 the organisation updating and selling these maps has become wholly owned government subsidiary with 95% of it’s income coming from electronic mapping and apps. However obviously this wasn’t always the case.

There are many scales but what I’ve found is that most websites. the most useful in historical research are the imperial coordinate systems.

Grid Reference Finder To The Rescue

See..

Grid Reference Finder.com

One Universal System

My daughter’s esteemed geography master once pointed out that no country on this planet is actually stationary.

Which begs the question: how can any converter work ?

Well luckily for you I’ve since found another website, which you may like, that answers that question. Indeed there are probably others. The author of this one goes to great pains to explain how a converter might do what it does without using a Cray Computer which is great bedtime reading if your partner has taken up watching snooker.

However they also warn that due to very slight tectonic non-linear shift (of course I knew that) the calculator given stopped being accurate after 2004 (Doh!).

So take a quick look and there’ll be a little test in the morning..

Moveable-Type Scripts Converter

But What About Tectonic Plates I Hear You Ask ?

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