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Ellipsoidal and Geoidal to Orthometric Height

  • 30-03-2011 11:27am
    #1
    El Siglo
    Registered Users, Registered Users 2 Posts: 3,803 ✭✭✭


    Alright folks,

    I've a number of datums given what I think might be in Ellipsoidal heights, although the crowd that gave them to me might have also used Geoidal heights (have to check with them). Anyway, if I have tide gauge data for a port (msl really) can I work out the orthometric heights fixed to this port?

    So instead of saying; "4.2 m above msl", could I work this out to something like; "5.9 OD (Sagar Island)"?

    Cheers folks, I would really appreciate the help.


Comments

  • #2
    RuggieBear
    Registered Users, Registered Users 2 Posts: 17,958 ✭✭✭✭RuggieBear


    might be of some use:

    http://www.pol.ac.uk/ntslf/tides/datum.html


  • #3
    Supertech
    Registered Users, Registered Users 2 Posts: 597 ✭✭✭Supertech


    OSi's converters might also be of some assistance http://www.osi.ie/calculators/converter_index.asp

    If you do a conversion on one of your values you may be able to compare it to a spot height or Bench Mark on the OS map for the area this will tell you whether the values are Ellipsoid or Geoid and once you know you can convert accordingly.


  • #4
    El Siglo
    Registered Users, Registered Users 2 Posts: 3,803 ✭✭✭El Siglo


    Supertech wrote: »
    OSi's converters might also be of some assistance http://www.osi.ie/calculators/converter_index.asp

    If you do a conversion on one of your values you may be able to compare it to a spot height or Bench Mark on the OS map for the area this will tell you whether the values are Ellipsoid or Geoid and once you know you can convert accordingly.

    Cheers for the advise, really appreciate it lads.

    Problem is, my site is in India and nobody over there has a clue of datums. What I had planned on doing was surveying my site to a temporary benchmark giving me relative heights with a differential GPS, then later on (taking into account subsidence etc...) I had planned on surveying that benchmark to a known tide gauge, giving me a fixed datum. However the geoidal model that they use in India is probably different to Ireland so I'll have to see what's going on there with that.


  • #5
    Supertech
    Registered Users, Registered Users 2 Posts: 597 ✭✭✭Supertech


    :o India ..... I'm afraid is slightly out my range !!


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