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What geoid model?

Discussion in 'GNSS & Geodesy' started by Kevin Samuel, Nov 12, 2013.

  1. Kevin Samuel

    Kevin Samuel 4-Year Member

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    If you are using single base rtk (no RTN, no static) for site surveys and boundary surveys, assuming bearings, and using the fugarwe button what geoid model do you use? Think... cowboy RTK (not best practice I know).

    I know the accuracy of the derived heights is dependent upon the accuracy of your base location.

    My questions are:

    Which geoid model would you use and why?

    Would you conduct any validity testing? How?

    I am Trying to convince the boss to at least load geoid12a. Is it worth doing given the above scenario or should I be content with geoid03?

    Advice?
     
  2. Jim in AZ

    Jim in AZ 4-Year Member

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    Geoid 12a
     
  3. Brian Allen

    Brian Allen 4-Year Member

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    > If you are using single base rtk (no RTN, no static) for site surveys and boundary surveys, assuming bearings, and using the fugarwe button what geoid model do you use? Think... cowboy RTK (not best practice I know).
    >

    Way back when, when this kinda thing was common (due mostly to lack of training) most of us barely knew what a geiod model was, let alone which one to use. That said, I only know of 1 company that is probably still practicing this way (no static/opus, assuming bearings, and fugarwe button) for boundary work.

    Please don't insult us cowboys by tying us to "not best practice", I ain't the sharpest knife in the drawer, but I try. ;-)


    > I know the accuracy of the derived heights is dependent upon the accuracy of your base location.
    >

    It also affects the precision of your horizontal. Every GPS job I do is tied to OPUS derived coords.



    > My questions are:
    >
    > Which geoid model would you use and why?

    I generally try to use the latest, granted I do very little work where elevations are critical.

    >
    > Would you conduct any validity testing? How?

    See above.

    >
    > I am Trying to convince the boss to at least load geoid12a. Is it worth doing given the above scenario or should I be content with geoid03?
    >

    Since it only takes a few minutes (OK maybe 30-60 for us who are IT challenged, ie cowboys), why not use the latest and supposedly the greatest? But then again, if your work isn't vertically important...........


    > Advice?

    Just load the latest and be done with it.
     
  4. Mark Mayer

    Mark Mayer 4-Year Member

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    Geoid 12a. For relative positioning.

    Sad that your boss is not just neglecting to update, but seems to be actively obstructing the process.

    It's so little effort to upgrade your positions by OPUS'ing the base position thereby allowing recovery of the control -to at least some degree- if the monuments are lost. So much is gained for so little effort.
     
  5. Kevin Samuel

    Kevin Samuel 4-Year Member

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    Good news! There was an overnight change of heart!

    Now I just need to figure out how to create a geoid sub grid for OR using foresight dxm.

    Thanks for the input/support guys!
     
  6. Kevin Samuel

    Kevin Samuel 4-Year Member

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    I totally agree with you Mark.

    Unfortunately we have a Topcon Hiper Lite RTK system with no onboard memory. I am looking forward to an upgrade... someday.

    We are limited to holding existing control or fugarwe-ing! :(
     
  7. GeeOddMike

    GeeOddMike 4-Year Member

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    FWIW,

    If working in the United States, using NAD 83 (2011) coordinates and wanting my heights to be in close agreement with NAVD 88, I would use the current hybrid geoid model, GEOID 2012a.

    I use the hybrid rather than the gravimetric model because the hybrid model accounts for the differences between NAD 83 and ITRF and, most importantly, includes a corrector surface generated from the computation of geoid heights from the difference between the GPS-derived ellipsoid height and differentially-determined NAVD 88 height at the point.

    I would verify the model in my work area by examining the map of the GPS on BM data set on the NGS site. I would review the spreadsheet showing details about the points used for the model in my area. Determine whether points in the area were excluded from the model creation and why.

    I could also determine whether there was a significant tilt or offset in the area by computing those parameters. I would examine the residuals of my solution. See http://geodesyattamucc.pbworks.com/w/file/fetch/65795664/lab8_2013.html for a Matlab-based solution.

    Of course, if there is a problem in my area (as there was in a number of areas in GEOID 2012) what do I do? I'd contact NGS at a minimum to report the problem. Ideally I'd make some OPUS-DB submissions from observations of published bench marks in the area. This provides NGS with the data needed to better model the problem area.

    Cheers,

    DMM
     
  8. Kevin Samuel

    Kevin Samuel 4-Year Member

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    No offense intended.

    I was thinking if shooting from the hip. :)
     
  9. Kevin Samuel

    Kevin Samuel 4-Year Member

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    Thanks for the feedback.

    That sounds like a great way to validate. A majority of the BMs in my area are in river canyons that are not friendly for GPS (narrow sky view).

    We have a smattering of other BMs around the county and I hope to do some outreach and get local surveyors to submit OPUS-DB solutions on survey marks when possible.
     
  10. exbert

    exbert 3-Year Member

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    I haven't used a Hiper Lite in years, but we could store the static internally with ours. Can you store the data in you data collector?
     
  11. Kevin Samuel

    Kevin Samuel 4-Year Member

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    Yeah, I checked into the internal memory for our Topcons... for whatever reason these models were built without internal memory. It doesn't exist.

    We are running TDS Survey Pro 4.7 on our Ranger. There is no way to log the static data to the DC. You can start data logging with the collector, but the receiver must have internal memory. Strange design, but it is the tool I have access to.
     
  12. exbert

    exbert 3-Year Member

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    I trust you have tried everything. But I did find this if it helps...

    http://www.youtube.com/watch?v=CiKlIJ9Dh0g
     
  13. Kevin Samuel

    Kevin Samuel 4-Year Member

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    That could prove useful, I would just need another data collector to make use of both units. I am not sure how to log static data at the base while performing rtk data collection.

    Thanks for the tip and I will explore further.
     
  14. MightyMoe

    MightyMoe 4-Year Member

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    It's rare that I will be on 2011 and Geoid12, those are usually projects that have no historical data. For instance the new FEMA maps in this area were generated from mapping that used 88 bench marks and Geoid 03 for control.

    So anything to do with elevations related to the flood plane will be controlled from those bench marks and Geoid03. The horizontal for those maps were taken from HARN points and those will be what I will use NAD83(93). OPUS is not allowed!!! Geoid12 raw from ellipsoid heights derived from OPUS-NO WAY!!!!!

    Also, most projects I work with are controlled in some fashion. For instance we are finishing a DOT project. They had control already along the 10 miles of highway and gave us a sheet outlining how it was done. HARN points, NAD83(93), leveled, and Geoid03 for panel points to control mapping. Rods and aluminum caps in concrete every 1500' or so. We occupied the control using the leveled values for elevation and applied Geoid03 and checked all the points. Very small errors along the project except for one which was disturbed by construction. Simple!

    Along comes the construction end of the project after our ROW portion (usually works in reverse) and the surveyor pukes OPUS and Geiod12 across the site!!!! I always use OPUS and the latest Geoid he says!!! LOL!!!! Glad I wasn't dealing with that mess!!!

    If I have a virgin project, and no ties to historical data I will use 2011 and GEOID12, but its unusual-somewhere out in the middle or nowhere-a couple of them this year.
     
  15. Pinetree

    Pinetree 3-Year Member

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    Lurking when I should be drafting

    Good thread. I learned a couple of things.

    Now I need to get back to work. Where the FUGWUZI?
     

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