Ellipsoidal and Geoidal to Orthometric Height

El Siglo

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.

RuggieBear

might be of some use:

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

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.

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.

Supertech

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