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I am taking observations in an area with zero survey pillars.My country, Ghana has not determined its geiod ellipsoid separation and for that reason i wanted to know the level of accuracy in the elevations given by the gps (in this case a sokkia radian IS) and also elevations given by google earth. There is another tool, elevation finder from freemaptools.com. I just want to know which is more likely to have a higher level of accuracy. 

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  • For those interested in Republic of Ghana historical info about geodesy http://www.asprs.org/a/resources/grids/06-2000-ghana.pdf

    (from PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING - Column Grid & Datums - Author Clifford J.Mugner)

    http://www.asprs.org/a/resources/grids/06-2000-ghana.pdf
  • Searching more detail about Gps Reference network in Ghana I've found two documents that can focus the argument , good also for a general geodetic culture around African plate.

    The first is this,

    http://ggim.un.org/knowledgebase/Attachment117.aspx

    where the topic of AFREF is discussed, and there is the detail for the GRN for Ghana ,

    but very interesting is this 

    http://d-nb.info/106798061X/34 ESTABLISHMENT OF GPS REFERENCE NETWORK IN GHANA author Yaw Poku-Gyamfi, that include Results of the transformation Parameters from Wgs84/Itrf05 to War Office 1926, also discussing about Ghana Vertical Datum definition and validating the global geoid model in Ghana.

    Regards

    CS

    • Thanks guys, i appreciate the help

  • Setup a monument, start observation, do at least 6 hours long (usually I do 10 hours), repeat 3 times (means 3 days) and send it to AUSPOS or the Canadian equivalent to get results from ITRF.

    Repeat this on another two monuments (placed roughly on an equal side length triangle with 5-8km long sides).

    Get another two receivers when all done and do a static observation using the three GPS on each points. Set take off point (or fix) the most accurate from the three. Play with adjustments.

    Now you'll get a picture about the accuracy of your GPS height, of course you have to tie it to your local geoid if need higher precision but at least you'd start any project in "your" system.

    (Also, use EGM2008 as mentioned earlier and WGS84/UTM for datum)

  • Thanks Anthony, I will have to assume an elevation in my case because I lack a tide station close by. I will have to travel over 200km to get access to one.
    • I've found interesting this doc here , full of useful information

      https://unbscholar.lib.unb.ca/islandora/object/unbscholar%3A7108/da...

       another interesting link (FYI)  is the Geoid repository http://www.isgeoid.polimi.it/Geoid/reg_mod.html

      For my opinion, Egm2008 is a good choice (also easy to work with) , usually very close to regional models, but trial test over local leveling benchmark are suggested.

      Don't know your project, in general you might think to carry out a static survey over a series of BM that can include your area ( BM with Ellipsoidal height and orthometrical Z) in order to calculate a transformation, for your area.

      Last, search info to your Ghana statal geodetic Institute, perhaps some project are still in movement and a solution can be provided.

      Hope this can help in some way.

      C

      https://unbscholar.lib.unb.ca/islandora/object/unbscholar%3A7108/datastream/PDF/download/citation.p…
  • Dear Mr. Ababio,

    An overly simplified description of the geoid model is a DEM of the differences between the ellipsoid (GNSS reference) and orthometric height (elevation from a level).

    GNSS doesn't know about UP. All points are X, Y, Z from the center of the earth, often expressed as Lat, Long, Hgt on an ellipsoid.

    Orthometric heights are perpendicular to the surface of the geoid, which is uneven dependent upon the varying density of the earth.

    One thing you may try is to find a tide station (or several) and see if it has been in use long enough to determine a mean sea level for that station(s) and using a surveyor's level, set several points around & through your project. Then using GNSS, determine the ellipsoid heights for those same points in your project. Using the differences you will be able to create a simplistic model of the geoid separation and be able to use it to estimate orthometric heights of other points within your project area using just GNSS & your model.

    If you lack an available tide station, you may assume an elevatin for one of your points and proceed from there knowing your elevations are based on an arbitrary assumption and not on geodetic references. This may be fine for some projects.

    JAC

  • Working in Kazakhstan North Caspian Sea we have a similar problem, i.e. no Geoid.

    We submit our gps session files in RINEX format to AUSPOS and CSRS, the heights returned by their online PPP are now relative to EGM2008. We then run some adjustment software like Trimble and use fixed orthometric heights on some of our points to adjust the new points.

  • Hey Joseph,
    Bit of an amazing question. I might have misread it. I'm reading two questions really.

    Q1 - How to manage the lack of a Geoid specific to your region.

    Q2 - Understanding the resolution and accuracy of online mapping tools.

    Answer1 - If you don't have a local geiod model for your country tied to your local height datum(which typically describes mean sea level and why you want to use a Geoid model) I'd suggest using a global one like EGM2008. It is likely your receiver / survey controller is interpreting this model when giving your orthometric heights(unless there is an established geiod more suitable that you are unaware of).

    Answer2 - You will be better off using a high resolution survey tool to measure the height of your monument rather than an online mapping tool. But!! It's really what you need the info for that can dictate what's suitable.

    Have look at section 4 in the link below. The link was Posted by Billy Brooks in relation to RTK surveying, but i think it's relavent regarding your comparisons of online mapping tools vs. survey measurement tools.

    https://www.e-education.psu.edu/natureofgeoinfo/book/export/html/1620

    Chapter 5: Land Surveying and GPS
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