Mean Sea Level?

Hi Henry,
The most current geoid model is the EGM 2008 and I understand it is very good. However, if my geology is correct, there are some pretty substantial subduction zones around the Fuji Islands. This can greatly impact any use of a model. A friend who was the national surveyor for Iraq was able to match EGM 08 across the entire country except in the northwest - colliding plates in that area create amazing gravitational anomalies. You can't use a model in those situations. The other questions to ask is does Fuji have a national vertical datum? A lot of the islands use a Mean Sea Level as their vertical datum and in the South Pacific the South Pacific Sea Level and Climate Monitoring Project is useful for determining new values. Samoa did a complete overhaul as outlined in a FIG presentation in 2013. I know that Australia and New Zealand have very extensive datums (vertical and horizontal) for the South Pacific region - any resources there?
I don't think I actually answered your question but in summary: 1) EGM 2008 and MSL vertical datums are not the same, you need to create a model from the local MSL vertical; 2) subduction zones make using a geoid model unpredictable; 3) many of the MSL datums are outdated due to sea rise and many are curently being updated; and 4) Australia and New Zealand have some excellent resources for that region.
a 2013 FIG presentation titled "Status of Geodetic Infrastructure in Fiji Islands" has an excellent description of the geodetic datum for Fuji showing the 18 Doppler stations set in 1979 and the efforts at that time to update with CORS and tying to the ITRF. Google that title and you will find a PPT. If you can't find it or open it I saved a copy and can email it to you - let me know.

Your best option in my opinion would be to find a benchmark or tidal station with a published value. Use the geoid model EGM08 in a relative mode rather than an absolute mode. In other words, you would be using the differences in geoid separation between points and hold fixed the published value.

Appreciate the input guys but you obviously have more knowledge about this then I do and I would need some step by step approach to achieve this... Any sites you know of that has this sort of information and allows me to follow some sort of workflow to achieve what you propose?
Just want to let you know that we do have one CORS Station in Fiji but do find it difficult getting any data from it since it is run by a regional organization that enjoys keeping all the information to itself.

Cheers.

Henry,
I just looked up the IGS Station: suva and it appears to not be functioning. If that is correct, good reason you're not getting data.
https://igscb.jpl.nasa.gov/network/site/suva.html
I assume you tried to contact the Ministry of Lands. I did find the survey report for the precise leveling and setting of benchmarks from 2005. You can find the information here: http://ict.sopac.org/VirLib/PR0389.pdf
Of course this won't help you if you are not on Lautoka.
As to relative elevations that John mentioned, you can use the EGM 2008, shoot several of the elevation benchmarks, and then adjust your collected points by the difference in order to meet the benchmarks posted elevation. So if the EGM 08 calculated the benchmark as 10.5 meters but the posted elevation is 9.8 meters, then the adjustment for your points is -0.7m. Best to verify against several benchmarks. The other option is to calibrate/localize vertically but you only create a elevation model within the points you collected and going beyond those points can really mess you up due to tilt. Best to raise or lower you collected points in reference to the published benchmarks as I illustrated.

Is this a great place or what?????

geonerd, post: 328866, member: 8268 wrote: Henry,
I just looked up the IGS Station: suva and it appears to not be functioning. If that is correct, good reason you're not getting data.
https://igscb.jpl.nasa.gov/network/site/suva.html
I assume you tried to contact the Ministry of Lands. I did find the survey report for the precise leveling and setting of benchmarks from 2005. You can find the information here: http://ict.sopac.org/VirLib/PR0389.pdf
Of course this won't help you if you are not on Lautoka.
As to relative elevations that John mentioned, you can use the EGM 2008, shoot several of the elevation benchmarks, and then adjust your collected points by the difference in order to meet the benchmarks posted elevation. So if the EGM 08 calculated the benchmark as 10.5 meters but the posted elevation is 9.8 meters, then the adjustment for your points is -0.7m. Best to verify against several benchmarks. The other option is to calibrate/localize vertically but you only create a elevation model within the points you collected and going beyond those points can really mess you up due to tilt. Best to raise or lower you collected points in reference to the published benchmarks as I illustrated.

You sir are a genius much appreciated! I have identified these bench marks, just need to confirm their exact locations as well as their elevation information...

Thanks again.

You may call her "sir" if you like, but I'd do it where she couldn't hear me.;-)