[Proj] geoid in Kalimantan, Indonesia

Wed Mar 5 22:44:14 PST 2008

Frans,
I tend to agree with Clifford. As a surveyor myself and having used the ProMark3's in anger I can say the manufacturers specifications leave the whole "accuracy depending on atmosphere" open to interpretation. When I read your post I assumed you'd performed a static differential survey with a pair of ProMarks to build a network including your tide gauges. If you have simply positioned your Promark above the tide gauge and observed for 20 or 30 minutes then your results are likely to be poor.

I have always used a rule of thumb that the vertical accuracy of a GPS is about 2.5-3 time worse than the quoted horizontal accuracy in RELATIVE terms. What you're trying to do is perform a gravity survey in ABSOLUTE terms. I've done countless surveys for irrigation over vast tracts of land. They key here is you need an accurate vertical geoidal model upon which to base your survey. Typically I've had the benefit of connecting to existing bench marks that were leveled with spirit or optical leveling techniques. Since both the spirit or optical levels are affected by gravity you have a great framework for basing your GPS survey.

Most of my work has been using Thales/Magellan DSNP Dual Frequency Long Range Kinematic GPS. They way we use this is to set up a base station in a convenient location.(Usually a high hill/mountain) within 40km of the job. There will be surveyors reading this saying I'm crazy, but believe me Thales mean LONG range kinematic.
Anyway I'd observe a network of previously established bench marks with Horizontal and vertical coordinates. That would establish my survey reference framework. After that any new or unknown points would then be coordinated relative to this framework.
In your case you may not have this network of existing "control". This is the challenge. I have transferred control using the above technique. What that means is as you survey points that are unknown as above they become "known". You can then pick up your equipment, transfer it to a convenient location closer to job site and use then new benchmarks as control for the new site.
This is not a job to be taken lightly. I'm also not too sure what the terrain is like in Kalimantan but I assume difficult to traverse.

From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of Clifford J Mugnier
Sent: Thursday, 6 March 2008 2:50 AM
To: PROJ.4 and general Projections Discussions
Subject: RE: [Proj] geoid in Kalimantan, Indonesia

EGM96 is the best worldwide geoid available, but a Promark is a single-frequency receiver and is a poor candidate for the third component.  I would estimate the vertical accuracy at about plus or minus 1 to 2 meters at best.  A dual frequency receiver can get fairly reliable verticals because the two frequencies allow proper modeling of tropospheric effects - an impossibility with only a single frequency receiver.  A short baseline differential technique (less than one kilometer with two simultaneous Promarks) is used for construction work, but certainly not even close to 10 kilometers muchless 30!

Clifford J. Mugnier, C.P., C.M.S.
LSU Student ASCE Chapter Faculty Advisor
and
National Director (2006-2008),
Photogrammetric Applications Division
American Society for Photogrammetry and Remote Sensing
and
Chief of Geodesy,
CENTER FOR GEOINFORMATICS
Department of Civil Engineering
CEBA 3223A
LOUISIANA STATE UNIVERSITY
Baton Rouge, LA  70803
Voice and Facsimile:  (225) 578-8536 [Academic]
Voice and Facsimile:  (225) 578-4474 [Research]
Honorary Life Member of the
Louisiana Society of Professional Surveyors
Member Emeritus of the ASPRS
Member of the Americas Petroleum Survey Group
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________________________________
From: proj-bounces at lists.maptools.org on behalf of Frans Buschman
Sent: Wed 05-Mar-08 05:03
To: proj at lists.maptools.org
Subject: [Proj] geoid in Kalimantan, Indonesia

Hello,

In a coastal region in East Kalimantan, Indonesia, we measured with a GPS promark3 system height differences of several water level gauges with respect to the WGS-84 ellipsoide and datum. The stations are about 30 km apart and we think that the precision of the measurements is within 10 cm.

>From the water level gauges we want to see in what direction water should be flowing, so we want the want to derive the height differences with respect to a geoid. For Kalimantan no detailed geoid seems to exist, so I would like to convert measured height differences on the WGS-84 ellipsoide to height differences on a global geoid. Which geoid is best for this purpose and how can I make the conversion? Could you give an estimate of the error you make with this conversion?

Thanks!
Frans

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______________________________________________________
Frans Buschman (PhD student)
Department of Physical Geography
Institute for Marine and Atmospheric Research Utrecht
Utrecht University
P.O. box 80.115 / Heidelberglaan 2
3508 TC Utrecht, the Netherlands
Room 211 in Jan Zonneveldvleugel
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mail: f.buschman at geo.uu.nl
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