The prospect of the upcoming move to our new head office has resulted in buzz of increased activity around the building in recent months. Just like when you move house, it’s been a good time to have a bit of a clear out and take stock of what’s been hiding under the proverbial bed or in the attic.
Well, part of that process has included cataloguing the many pieces of antique surveying equipment that have been accrued by Ordnance Survey over the past 200 years. Some of the items have played an historic role in the birth of modern map making in Britain and are irreplaceable.
To understand more about some of this fascinating equipment, I caught up with Ken Lacey, a surveyor by trade who now works in our education team. Ken was kind enough to give me a tour of what is rapidly turning into an Aladdin’s cave of cartographic memorabilia, with two pieces being of particular interest.
Here’s what Ken told me:
Ramsden 18” Theodolite
This beautiful piece of kit was made by Jesse Ramsden and bought by Ordnance Survey in 1795.
It is 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.
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.
It was used in 1826 for obtaining the precise direction of the Lough Foyle base line, and from that time at Principle Triangulation stations throughout the British Isles.
This famously included 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!
Newlyn Tide Gauge
Ever wondered how we can give a height of an object above sea level? Think about it, the sea is rising and falling throughout the day, year by year and at certain times of the months and seasons we get a higher tide or a lower tidal point.
To calculate Mean Sea Level Ordnance Survey needed to record all instances over the full range of possible variations. That required at least one year of observations for a reasonable determination, but in practise it was calculated over a number of years. From these observations it was then possible to establish the point of mean sea level and from that then calculate the difference in height from this point to any other fixed location.
To take all the readings required to calculate Mean Sea Level, you needed a tide gauge. Examples were set up Felixstowe (1913), Newlyn in Cornwall (1915) and Dunbar (1917). Subsequently it was decided to solely rely on the tide gauge at Newlyn, which is the one we’ve got here in Southampton, stored out of use.
It was chosen over the others because it was situated in an area of stable granite rock and because the gauge was perched on the end of a stone pier at the harbour entrance it was exposed to the open Atlantic.
It was not therefore liable to be influenced by the silting up of the estuary or river tide delays, as was the case at the previous gauge at Liverpool.
It worked by recording the rise and fall of the sea with two floats attached by chains. The variations were recorded on paper attached to the rotating drum suspended from its centre.
Every height measurement in the country has been calculated based on the work of this machine.
Responsibility for the Newlyn Tide Gauge continued to rest with Ordnance Survey who provided a full time observer until 1983 when the station was handed over to the Institute of Oceanographic Sciences.
Why not check out some of our other stories from behind the scenes?