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Vertical Datumns, OPUS, FEMA

Discussion in 'GNSS & Geodesy' started by Larry Best, Apr 20, 2017.

  1. Larry Best

    Larry Best 6-Year Member

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    I'm trying to get a LOMA through and FEMA tells me that I must use the Local Tidal Datum, whish is VIVD. The LOMA form says I will need to attach the Datum Conversion. I got my elevation from OPUS. But what Datum is the OPUS ORTHO HGT? Where can I find conversion factors? There is only one Bench Mark listed on the NGS website for the island (DL3633) and it's destroyed. A year ago I got a OPUS RS solution for a disk marked "National Ocean Service 1381A 2006" that was off 0.034 M from some published value, but I can't find that published value again. Now this is for a house 100 feet above the sea on the side of a steep slope, so from a practical matter, it doesn't matter, but I need to get this through and I need to know more about datums.
     
  2. GeeOddMike

    GeeOddMike 6-Year Member

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    IMG_0145.jpg The two links below provide information about vertical datums and other related topics.
    https://www.meted.ucar.edu/training_module.php?id=1099&tab=04#.WPjSEpH3afA

    https://www.ngs.noaa.gov/corbin/class_description/NGS_Video_Library.shtml

    This is a link to a tool allowing the user to find information about survey monuments and CORS using a graphical interface:

    https://geodesy.noaa.gov/NGSDataExplorer/

    Searching St Croix with the tool found a number of monuments.

    As for the question of the datum associated with your OPUS solution, it should be indicated in the solution or associated metadata.

    Be advised that there are sites with data from both the National Geodetic Survey and CO-OPS. There should be a link on the datasheets allowing access to the other site's data.

    There is a handy tool, VDATUM, that allows transformation among height systems. It is on the NGS web site.
     
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  3. Larry Best

    Larry Best 6-Year Member

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    I'm on St. John. St. Croix is 40 miles south.
     
  4. GeeOddMike

    GeeOddMike 6-Year Member

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    IMG_0147.PNG


    Same process, different island...

    For tidal data on 1381 A (actually 9751381A) go to: https://tidesandcurrents.noaa.gov/benchmarks.html?id=9751381

    There are other tidal benchmarks published by CO-OPS in the area as you can see on the map at the linked site.

    Read the descriptive information on the NGS data sheet for how VIVD is described. This is the vertical datum origin point for VIVD09!
     
  5. GeeOddMike

    GeeOddMike 6-Year Member

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    IMG_0150.PNG While located via the link provided above, this graphic might be more helpful. Drat it's in those pesky "feet"...
     
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  6. Larry Best

    Larry Best 6-Year Member

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    Thank You so much Mike!
    That NOAA bench mark site was what I found before but somehow lost.

    From the OPUS report, I think I am getting NAD83:


    All computed coordinate accuracies are listed as 1-sigma RMS values.
    For additional information:
    http://www.ngs.noaa.gov/OPUS/about.jsp#accuracy

    USER:
    larrybest@bestwinters.com DATE: July 12, 2016
    RINEX FILE: 9221192q.16o TIME: 20:23:53 UTC


    SOFTWARE: rsgps 1.37 RS94.prl 1.99.3 START: 2016/07/10 16:19:10
    EPHEMERIS: igr19050.eph [rapid] STOP: 2016/07/10 18:19:10
    NAV FILE: brdc1920.16n OBS USED: 8070 / 8484 : 95%
    ANT NAME: CHCX90D-OPUS NONE QUALITY IND. 33.68/ 14.83
    ARP HEIGHT: 1.5240 NORMALIZED RMS: 0.414


    REF FRAME: NAD_83(2011)(EPOCH:2010.0000) IGS08 (EPOCH:2016.52383)

    X: 2586136.442(m) 0.009(m) 2586135.773(m) 0.009(m)
    Y: -5477028.296(m) 0.018(m) -5477026.452(m) 0.018(m)
    Z: 1991840.047(m) 0.011(m) 1991839.968(m) 0.011(m)

    LAT: 18 19 5.64262 0.007(m) 18 19 5.66015 0.007(m)
    E LON: 295 16 32.47017 0.006(m) 295 16 32.47638 0.006(m)
    W LON: 64 43 27.52983 0.006(m) 64 43 27.52362 0.006(m)
    EL HGT: -41.198(m) 0.022(m) -43.077(m) 0.022(m)
    ORTHO HGT: 1.045(m) 0.022(m) [ H = h-N (N = GEOID12B HGT)]

    Using the VDATUM tool, I get:

    Exception = java.lang.IllegalArgumentException: Unsupported datum transforms: from [horz=NAD83, vert=NAVD88] to [horz=NAD83, vert=VIVD09]

    Any Adeas?
     
  7. base9geodesy

    base9geodesy 4-Year Member

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    In USVI OPUS returns heights that are related to VIVD09.
     
  8. Larry Best

    Larry Best 6-Year Member

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    But the LOMA form says "Indicate the Datum (if different from NGVD29 or NAVD88 attach datum conversion)"

    What do I attach?

    FEMA sent this back to me to correct it. When I called them they said call the Local Flood Plane Administrator. How much help is that? Beerleg is my only hope.
     
  9. Larry Best

    Larry Best 6-Year Member

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    It doesn't say so. It says NAD 83
     
  10. SPMPLS

    SPMPLS Member

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    I do not believe that there is any datum conversion to perform/attach. VIVD09 is the official vertical datum. Your OP says that FEMA requires you to use VIVD and Mr. Doyle (base9geodesy) has told you that the ortho height listed on the OPUS report (1.045 m +/- 0.022 m) is a VIVD height. You are already on the FEMA required datum.
     
  11. SPMPLS

    SPMPLS Member

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    NAD 83 is not a vertical datum. The ellipsoid height can only be related to a vertical datum through the application of a geoid model, in this case 12B.
     
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  12. Larry Best

    Larry Best 6-Year Member

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    Look, I don't care about any of this , except how to get this LOMA passed. FEMA thinks NAVD is a datum, even if SMPLS doesn't think so. but it is not acceptable for the Virgin Islands. That's why FEMA sent it back to me. It must be VIVD09. And it must have a conversion attached. My alternative is to tell my customer to keep paying insurance for the flood zone.
     
  13. GeeOddMike

    GeeOddMike 6-Year Member

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    IMG_0151.jpg

    A few quick comments.

    1. I hope that NGS will add a FAQ about the issue of what orthometric heights are provided in the Virgin Islands. I cannot find reference to it providing VIVD heights for solutions there.

    2. As the horizontal position is clearly labeled NAD83 (2011) epoch 2010 and the geoid- ellipsoid separation is indicated to be from GEOID12B, I am left to conclude that the hybrid geoid must incorporate the 1.079 height mentioned in the data sheet screen capture above. Again, there should be more reassurance provided that the ortho height does reflect VIVD09.

    3. Finally, note the uncertainty in the computation of the GEOID12B value. It is 3.798 meters at the 95% CI. 3.798 METERS!

    Finally I will waive my usual consultation fee and perform an onsite visit from the nearest 5 star resort of your choosing.
     
    Last edited: Apr 20, 2017
  14. SPMPLS

    SPMPLS Member

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    I did not say that NAVD was not a vertical datum. Clearly it is since it stands for North American Vertical Datum. You said that NAD 83 was a vertical datum and I told you that it was not. You can keep arguing, but base9geodesy is the retired NGS Chief Geodesist, so when he said the ortho height you are getting on the OPUS Report is referenced to VIVD09, my money is on him being correct.

    You are already on VIVD09. There is nothing to convert.

    Maybe you should email FrancisH for assistance?
     
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  15. Larry Best

    Larry Best 6-Year Member

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    I have no interest in arguing with experts on geodesy, a topic that I obviously have very little knowledge of.
    I apologize for leading this thread in that direction.
    Perhaps there is someone reading this who knows how to get paperwork through FEMA who can comment on my dilemma.
     
  16. SPMPLS

    SPMPLS Member

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    https://www.ngs.noaa.gov/GEOID/GEOID12B/GPSonBM12B.shtml

    GEOID12B is based in VIVD09 in the US Virgin Islands. Therefore, I do not think it possible that OPUS would return an orthometric height solution referenced to anything other than VIVD09 when using 12B. It would not be possible (or valid) for NGS to compute a NAVD 88 (or NGVD 29) ortho height for that geographic location using 12B.
     
  17. Bill93

    Bill93 6-Year Member

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    Last edited: Apr 20, 2017
  18. Larry Best

    Larry Best 6-Year Member

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    The solution to this may be to appeal to FEMA. I called their Assistance Center line again and again and pleaded for help. I was assured that someone will look into this and help me out.
     
  19. GeeOddMike

    GeeOddMike 6-Year Member

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    IMG_0152.jpg

    Note that the text above verifies base9geodesy's statement that OPUS provides an orthometric height based on an NAD83 ellipsoid height and the hybrid geoid model which takes into account the relationship of VIVD09. In other words, the hybrid model, GEOID12B, includes transformations and "corrections" to allow users to approximate orthometric heights from NAD83 ellipsoid heights (h - H - N = 0). Unfortunately, as shown in the computation posted previously there is a +3 meter uncertainty for the value at your location.

    Copy the text above and use it in your report. That said, the OPUS output should include some additional text covering this situation. I always like volunteering other people to do more work.

    BTW, base9 was never the Chief Geodesist but Chief "Geodetic Surveyor." Big difference.
     
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  20. spledeus

    spledeus 5-Year Member

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    Find the document that states the OPUS solution Ortho Height is VIVD09. Note the geoid issue and that it has little to no relevance as your clients' house is more than the 95%error above the bfe.
     

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