[Proj] RE: geoid in Kalimantan, Indonesia

Frans Buschman F.Buschman at geo.uu.nl
Fri Mar 7 04:00:09 EST 2008

Hello again,

Thanks for your useful replies, Clifford and Andrew. The best geoid 
available is thus the EGM96 and I understand that this accuracy is at 
best 1 m vertically worldwide.
Indeed we did not have the luxury of having control sites and indeed it 
was difficult to decrease the spacing due to dense vegetation or 
mangrove swamps. We have just used two GPS promark 3 receivers 
simultaneously at locations maximally 30 km apart to measure the 
relative height differences. You mention that the vertical accuracy of 
this system over these distances is 1or 2 m at best. So, combining these 
errors it seems more reliable to average the tidal gauges and assume 
that they have the same mean level!

One question, out of curiosity, remains: we realised that the distances 
between the stations were long for this GPS promark 3 system, so we 
performed the measurements during the night since we assumed that the 
least ionospheric error would be made then. How much does this improve 
the accuracy?

Thanks again,

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>    1. RE: geoid in Kalimantan, Indonesia (Andrew Williams)
> ----------------------------------------------------------------------
> Message: 1
> Date: Thu, 6 Mar 2008 17:44:14 +1100
> From: Andrew Williams <AWilliams at rapidmap.com>
> Subject: RE: [Proj] geoid in Kalimantan, Indonesia
> To: "PROJ.4 and general Projections Discussions"
> 	<proj at lists.maptools.org>
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> 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,
> Department of Civil Engineering
> CEBA 3223A
> 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
> ======================================================
> http://www.asprs.org/resources/GRIDS/<https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir!
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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
> --
> ______________________________________________________
> 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
> Phone: +31 - (0)30 2532778
> Fax: +31 - (0)30 2531145
> mail: f.buschman at geo.uu.nl
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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
Phone: +31 - (0)30 2532778
Fax: +31 - (0)30 2531145
mail: f.buschman at geo.uu.nl

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