9
Jul
2010
0

Exploring our surveying antiques

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" Ramsden Theodolite

The 18″ Ramsden Theodolite

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.

The Newlyn Tide Gauge

The Newlyn Tide Gauge

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.

One of the floats that was attached to the bottom of the gauge

One of the floats that was attached to the bottom of the gauge

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?

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9 Responses

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  2. Khagendra Thapa

    The above write up does not give the whole story of the measurement of the Mean Sea Level(MSL). In reality, the tide gauge observations must be taken for a period of 18.6 years which is the period of nutation. But what is nutation? Well, as you may know the spin axis of the earth is tilted by 23.5 degrees, and this axis is moving and it completes a revolution in 25765 years. The movement is, however, not uniform. There are disturbances to the motion and it is mainly caused by the moon and it is called nutation. The period of nutation is 18.6 years. Hence, we need to take the tide gauge observations for 18.6 years before we can establish the MSL.
    Please note that it is a rather simplified explanation of nutation. Both precession and nutation exist because the earthe is flattened at the poles by (1/298.26). Had there been no flattening, precession and nutation would not exist.

  3. Moshe Fogel

    I noticed your Ramsden 18” Theodolite. I have surveying company in Israel
    “M.Fogel surveying LTD”. I collect antique instrument for my small museum. I would like to know if this instrument is for sale? Also if you have other antique theodolites / compasses for sale ?

    Kind regards, Moshe

    1. Hi Moshe

      Thanks for your query on the theodolite. We keep our theodolite on display at our head office as we consider it an important part of our heritage as Great Britain’s national mapping agency, so I’m afraid it isn’t available for sale. Similarly, any other historic instruments we keep are also important to our organisation and are items that we wish to maintain at this time.

      Kind regards
      Gemma

  4. Dear Ordnance Survey,

    I am looking to source a theodolite, particularly one which may have been used in the late 18th century. I am very keen to know if you have any theodolites in your collection and if not, if you are able to point me in the right direction for one.

    I am part of a team producing a new three-part television series for Channel 4 Features presented by Alan Titchmarsh which will showcase master landscape designer, Lancelot “Capability” Brown. The series will celebrate his enduring legacy in landscape design and uncover his fascinating yet largely unknown plans for the home of the Duke and Duchess of Rutland, Belvoir Castle near Grantham. This landmark series is being made for More4 and Channel 4 Television by Spun Gold TV, with broadcast expected late 2015.

    As part of this series, we are looking to talk about how people would have measured the landscape in Brown’s time and even demonstrate this if at all possible. I am looking to talk to someone who may have knowledge of theodolites, and even access to one which we could look at if at all possible. There is no obligation for anyone to be filmed or take part in the series, I am simply researching the instrument at this stage so would like to talk.

    Please do email me to discuss your collection and the series.

    Very best wishes,

    Natalie

    1. Hi Natalie

      I understand you’ve also been in touch with our Press Office about this and they’ll be the right people to look into this for you.

      Thanks, Gemma

  5. nigel cook

    The tide gauge on
    https://www.ordnancesurvey.co.uk/blog/2010/07/surveying-our-antiques/
    looks like a Cary Porter one, the same make used in Southampton /Town Quay in the early 1900s.
    Would it be possible for me to view it close up?
    I live in Southampton.
    There was an extreme tide , the level of which, seems to have got into the records with an error , due to a slippage in the top clamp of the recorder.
    I’d like to be able to confirm , that the top clamp on the traverse and pen carrier , could have so malfunctioned, ie it is possible to have slipped with only the forces available in a float gauge set up.

    1. Hi Nigel

      I’ll take a look into this and get back to you. It will take a couple of weeks as the person who manages our artifacts is out of the office. I’ll find out if we retained the item when we moved to our new head office at the end of 2010, and whether it is available for viewing.

      Thanks, Gemma

    2. Hi Nigel

      I’ve now checked with the team and the gauge is now in secure storage in Warwickshire, and not at our head office. If you would still be interested in attending, drop us an email to customerservices@os.uk and they’ll be able to pass your request to the team to see what’s possible.

      Many thanks
      Gemma

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