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Thread: Trig Points

  1. #1
    Ultra King alexander rae's Avatar
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    http://blog.ordnancesurvey.co.uk/201...s-young-today/

    Happy birthday to Trig Points

    So whats the difference between the Trig Point in the link above and the one in the link below,apart from the fact that one is OS and the other is an Admiralty one, why are there 2 types ? , one has groove and the other is a flat plate.

    http://www.flickr.com/photos/3438428...7626385754569/

  2. #2
    ‹bermensch
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    Alexander

    I believe the reason for the difference in design is because the trig point on Slaggan Bay is not an Ordnance Survey trig point (the Explorer map does not depict a trig point at this location).

    All OS trig points conform to a standard design. The only difference between any of them is that most are constructed in concrete but some in local stone.

    The trig points that we all know and love were built between 1936 and 1962 as a result of the retriangulation survey, known as OSGB36. They were all constructed by OS engineers with the assistance of labour recruited locally. It was an heroic task and makes fascinating reading.

    I know nothing about hydrographic surveying other than the fact that it is/was the responsibility of the Admiralty. The plate to which you refer makes it clear that this structure was erected by the Admiralty.

    Hope that this helps

    Hugh

  3. #3
    Ultra King Paddy Dillon's Avatar
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    Some trig point lore...

    Many people think that the height given on maps is the height on top of the trig point. Actually, it's the height on the bracket near the ground, on the horizontal line on top of the 'crow's foot' mark. Even that height is artificial, as it's calculated from another reference point, a brass button set into the ground, with the trig point completely enclosing it.

    Not every brass button set into the ground got a trig point raised on top of it. Old Ordnance Survey records mention lots of these places, with cryptic notes about where they are located. As they're at ground level, they're easily obscured. Some of the notes mention some of these locations as being in churchyards, adding... "The Vicar knows where." Ah yes... well he'll be long dead!

  4. #4
    ‹bermensch
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    Paddy wrote:

  5. #5
    Ultra King alexander rae's Avatar
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    A lot of info there Hugh.

    http://www.naval-history.net/xGM-Chr...urvey-Cook.htm

    HMS Cook ( not that I know a lot about it ) was surveying in the Minch area ( link above ).

    A lot of work even erecting the one a Slaggan, I thought it may have been erected due to the Loch Ewe Depot but HMS Cook was also in the Skye area and coast so that idea falls flat.

    As someone mentioned the Admarilty ones don't have the "Groove" in it that the OS ones do,the triangle on top of the Admarilty one is in the middle, this one was on top of Ploc an Slaggan and i cant see it marked on the Landrangermap.

  6. #6
    Ultra King Paddy Dillon's Avatar
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    Very true Hugh, but a 'triangulation point' is a triangulation point no matter when it was triangulated, and regardless of whether it has a pillar built on top of it. Over the years, of course, a 'trig point' has come to refer to the mini-obelisk pillar on top of the actual point.

    The Ordnance Survey never turned out the way it was supposed to. When they realised that their original maps were a mess, they started all over again. The actual starting point was a 'measured mile' on some flat ground near the mouth of Lough Foyle in what is now Northern Ireland. The Ordnance Survey was not only going to make great maps, but they were tasked with gathering together history and folklore too. Unfortunately, the first (and only) parish they did in this manner almost wiped out the entire funding for the British Isles... so after that they just stuck to maps.

    (Edited to add to the above... this was another 'mess'... pre 1936!)

  7. #7
    ‹bermensch
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    Alexander

    Thanks for the info about HMS Cook.

    I'm still puzzled by your 'trig point'. The photo seems to show faint traces of the grooves used to accommodate a theodolite as use by OS surveyors.

    The triangle in the centre does not seem to be large enough to accommodate a regular theodolite. OS surveyors had a special tool with which to open the bronze plate on the top of the pillar so perhaps the hydrographers had something similar.

    There are no unique marks on the plate in the photo so perhaps each survey ship carried a supply of them to be fixed as required. (This is mere speculation. It would be nice if there was an OM hydrographer who could enlighten us.)

    I can't think of another explanation but, as I stated earlier, I'm wholly ignorant of the survey methods used by hydrographers (but Captain Cook is one of my heroes!).

    Hugh

  8. #8
    Ultra King alexander rae's Avatar
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    I was struck by the amount of concrete that would have had to have been carried up to the position they chose,there was a bog nearby which could have supplied water so at least the concrete would have been carried up dry, making it a bit lighter.

    I noticed that (when looking at the date on the plate ) the 51 was a different size from the 19, so i take it they had a box of themin the ship and just added the year as needed.

    I was going to try and contact the website the ship is on,but havent got round to it yet , there should be members of the crew around somewhere,I thought the photos may bring back memories.

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