Vertical Datumns, OPUS, FEMA

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.

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

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!

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

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#accurac y'"> http://www.ngs.noaa.gov/OPUS/about.jsp#accuracy

USER: [email protected] DATE: July 12, 2016
RINEX FILE: 9221192q.16o TIME: 20:23:53 UTC

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?

In USVI OPUS returns heights that are related to VIVD09.

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.

base9geodesy, post: 424551, member: 7189 wrote: In USVI OPUS returns heights that are related to VIVD09.

It doesn't say so. It says NAD 83

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.

Larry Best, post: 424561, member: 763 wrote: It doesn't say so. It says NAD 83

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.

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.

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.

Larry Best, post: 424570, member: 763 wrote: 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.

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?

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.

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.

Definition:
https://www.ngs.noaa.gov/datums/vertical/

St. John reference Bench mark
https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=DL3636

Do they have flood maps of the area? What were those tied to?

They want computations, give them some worst-case computation. Take ellipsoidal height plus Geoid12B plus geoid uncertainty, and note that this leaves plenty of margin at the ~100 ft property in question. They might buy it.

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.

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.

Larry Best, post: 424580, member: 763 wrote: 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.

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.