Map history at OS with Antiques Road Trip

If you were watching Antiques Road Trip yesterday afternoon, you’d have seen antiques expert Paul Laidlaw visiting our Southampton head office. The modern building we’ve been in since 2009, is a far cry from our first home at the Tower of London, and even the military barracks which became our first Southampton head office. But, despite being a digital data company in a state of the art building, there are still many nods to our mappy heritage to be found.

Our CEO Nigel Clifford showed Paul our first map, and an early theodolite while filming for the programme. Here’s a bit more about them:

Paul Laidlaw (left) and Nigel Clifford (right) in from of an 1801 map of Kent and the Ramsden 18″ theodolite

Ramsden 18” theodolite

On 21 June 1791, the Board of Ordnance purchased a new Ramsden theodolite, and this is seen as the foundation of OS. The theodolite at our head office was also made by Jesse Ramsden in 1795. It’s made to the same pattern as the two larger 36” theodolites, also made by Ramsden, and used for the construction of the Principal Triangulation, the very first national triangulation programme to cover the whole of Great Britain which began in 1791.

Triangulation is a mathematical process that makes accurate map making possible. It works by determining the location of a point by measuring angles to it from known points at either end of a fixed baseline. In later years, theodolites were secured to the top mounting plate of OS trig pillars and angles were then measured from the pillar to other surrounding points.

The 18 inches refers to the diameter of the horizontal measuring circle. This circle is where you can read the angle between two points of interest. To read this angle you need to view graduated marks through 3 microscopes.

This theodolite was used in 1826 for obtaining the precise direction of the Lough Foyle base line, and from that time at Principle Triangulation stations across Great Britain. This famously included being used on a platform over the top of the cross on St Paul’s Cathedral in 1849. When you consider that its dimensions are 540x720x550mm and it weights 28kg that is no mean feat!

1801 map of Kent

Our first map. And we wouldn’t be here without it. Our original purpose was to create a map that would help our military to defend and protect the nation. England’s most south-easterly county, Kent, was the area most vulnerable to French invasion, so that was our subject – highlighting the county’s communication routes, and including elaborate hill shading to interpret the landscape precisely for those men at arms.

1800s Kent

The map was completed at the Tower of London Drawing Room and produced at the one inch to one mile scale. It was printed by William Faden of Charing Cross, a leading cartographer and map publisher at the time.

OS today

The modern equivalent to the network of trig points is the OS Net network of 110 Global Navigation Satellite System (GNSS) receivers. Our surveyors use OS Net and GNSS technology, rather than theodolites, everyday to instantly position new map detail to within a few centimetres.

Find out more about how we survey today.

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