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Question re OS Malin datum marked on site plans

  • 14-11-2023 10:15pm
    #1
    dmcdona
    Registered Users, Registered Users 2 Posts: 1,880 ✭✭✭


    Built a house in Galway 3 years ago - about 100m from the sea. Site GPS survey drawings by an engineer marked on the elevations with the datum of O.S. Malin.

    As a sailor, I know how to "compare" tide heights from different locations so should be able to "convert" the engineer elevations to the actual height of the foundations above the local water level. Key word there is should...

    Anyone got an easy answer?

    Cheers



Comments

  • #2
    Mellor
    Registered Users, Registered Users 2 Posts: 39,902 ✭✭✭✭Mellor


    https://www.boards.ie/discussion/2058322621/question-re-os-malin-datum-marked-on-site-plans

    Malin is average sea level.



  • #3
    MrMusician18
    Registered Users, Registered Users 2 Posts: 7,755 ✭✭✭MrMusician18


    All (modern) levels in Ireland use Malin as the ordinance datum. If you have the water level charts that are referenced to Malin then there is no conversation needed.

    Worth noting that the elevation value between different years of surveys are not directly comparable as it may use a different geoid. The difference between osgm02 and osgm15 is almost 500mm in west Galway!



  • #4
    dmcdona
    Registered Users, Registered Users 2 Posts: 1,880 ✭✭✭dmcdona


    Thanks folks.

    After asking the question here, I mulled it over a bit and figured that no conversion was necessary.

    If the house foundation is at 13m OS Malin on the drawings (ie 13m above mean sea level) and my local tide is say 3m above mean sea level, then the house foundations are 10m above the current sea level...

    Thanks for confirming - much appreciated.

    Interesting point on the geoid - I'm guessing that if there was a 500mm difference between OS Malin and Galway, with our elevation at 13m, it's not that big a deal for us.



  • #5
    MrMusician18
    Registered Users, Registered Users 2 Posts: 7,755 ✭✭✭MrMusician18


    https://www.boards.ie/discussion/comment/121372638#Comment_121372638

    Just to explain (it doesn't matter in your instance, but it might help someone else), all modern maps an surveys use Malin as the ordnance datum. All modern surveys also calculate heights based on the geoid. The geoid gets updated and refined from time to time.

    You can run into problems when you need to compare a survey taken in say 2005, Vs one taken yesterday. For the same parcel of land, the heights of on the survey would be different. For most of Ireland it's not a lot (<100mm), but there was a big adjustment for west Galway.

    The other thing to note is that if the map has heights in feet, it is based off the poolbeg datum (which was based on the low water mark at Poolbeg lighthouse iirc). To convert from poolbeg to malin you need to add 2.7m and that gives you the approximate corrected height (notwithstanding the geoid issue also)



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