trig pillar


An OS history of the theodolite

If you’re an avid reader of our blog or have some surveying knowledge, you may well have heard of a theodolite before. A key instrument in the mapping of Britain (and pretty much everywhere else!), a theodolite is a simple concept built into a precision instrument – a telescope with graduated horizontal and vertical circles attached to enable angles to be measured. Intrigued by this implement, we decided to find out more… Read More


Britain’s top trig-bagger adds FBMs to his haul

Some of you may remember Rob Woodall, we shared his story in 2016 as he completed a 13-year mission to bag 6,190 trig pillars across Great Britain. Not content with that, Rob’s now added all 201 fundamental benchmarks (FBM – see photo below) to his haul. Rob fills us in on his challenge and talks about other OS survey marks he likes to bag along the way…

My first FBM was Wolsingham in lovely Weardale, in 2004. A group of us get together once a year, and that year the gathering was hosted by a couple who live in Wolsingham and they put together an itinerary which included trig pillars (my main interest at the time) and also mysterious things such as non-pillar flush brackets (NPFBs, which are FBs on structures other than pillars, such as houses, churches, bridges etc) – and the local fundamental benchmark. So I started ticking off NPFBs and FBMs too.

It’s taken 13 and a half years to bag 201 FBMs (including all but 6 of the destroyed ones – which I’ll get round to eventually) – coincidentally about the same time it took to bag 6,190 trig pillars – which is not great productivity.  However, the last two were Patrington (which we all thought was destroyed until it turned up in 2016 when the householder took out a big laurel tree in their garden) – and Windsor Castle which we all assumed was out of bounds. Eventually a friend sent off a letter to the Royal Collection Trust to see if we could visit the FBM, and the answer  was yes! – provided HRH was away at the time. So with no particular plan, I ended up finishing the list this year. Read More


#GetOutside with some trig-bagging this summer

Worcestershire Beacon photo by #GetOutside champion Dan Grant

Worcestershire Beacon photo by #GetOutside champion Dan Grant

Hands up if you’re looking for some free family activities over the summer holidays? We thought so…how about a spot of trig bagging to get the family outside? This year we’ve been celebrating the 80th anniversary of the trig pillar, those concrete pillars that are often found at the top of hills and create a handy photo opportunity.

Once a key part of our surveying network, and since superseded by GNSS, they stand tall and mark the summit of many a walk. With around 6,000 still standing around Britain you stand a fair chance of spotting one when you’re out exploring, and you can spot them on your map as the small blue triangle with a dot in the centre. Read More


A history of the trig pillar


The trig pillar network

Yesterday marked 80 years since the trig pillar was first used in the retriangulation of Great Britain on 18 April 1936. On that day, a group of surveyors gathered around a white concrete pillar in a field in Cold Ashby and began the retriangulation of Great Britain.

We’re celebrating by sharing the story of the humble trig pillar, still much loved by walkers today, and giving you the chance to join our celebrations with The Trig Pillar Trail Challenge. But what is the background to the trig pillar?

Read More


Celebrating the Hotine contribution

Amidst the joy of our Christmas celebrations this year, staff at Ordnance Survey will be remembering the contribution made to the national mapping agency by Brigadier Martin Hotine CMG CBE RE.

Brigadier Hotine served in the Royal Engineers for many years and saw active service in both the First and Second World Wars.  However, it was during 1934–1939 when he worked at Ordnance Survey heading up the catchily-titled ‘Trigonometrical and Levelling division’ when he made the contribution which we’ll be remembering.

Hotine was the person responsible for the design, planning and implementation of the re-triangulation of Great Britain on which Ordnance Survey maps are still based.  During this inter-war period, he designed the iconic and much-loved Trig Pillar which is still found now on many hilltops and in the countryside across Great Britain.

Read More


Happy birthday to the Trig Pillar – 75 years young today

On a clear, crisp spring morning in 1936, a group of men gathered around a strange, pale obelisk in the middle of an unremarkable field in Cold Ashby, Northamptonshire. Those men were there to begin the greatest undertaking Ordnance Survey had attempted since the early 19th century.

That shining white monolith would now be instantly recognised by any walker, hiker or geography pupil. It was of course a Trig Pillar, and today, 18 April, marks 75 years since the day when they were first used in anger at the beginning of the Retriangulation of Great Britain.

Where it all began - Cold Ashby

Where it all began – Cold Ashby

Trig Pillars now evoke the kind of sentimentalism of something quintessentially British (although there are equivalents in other countries), but at the time they were part of a state-of-the art network built to literally re-write the map of Britain. The Retriangulation was an enormous task and lasted up until 1962 (with a break for World War II), the impact of which we still live with today.

Triangulation is basically 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 and in this case, those known points were the 6,500 Trig Pillars erected across the country. In practice, a theodolite would have been secured to the top mounting plate and made level. It would then be directly over the brass bolt underneath the pillar. Angles were then measured from the pillar to other surrounding points. For the highest accuracy primary points in the Retriangulation, many rounds of angles would have been measured with the observations taking several hours.

But why was the Retriangulation needed?

Read More