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Larry Best
(@larry-best)
250+ posts Registered

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.

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Posted : April 20, 2017 6:50 am
GeeOddMike
(@geeoddmike)
500+ posts Registered

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.

DMM

The power of instruction is seldom of much efficacy except in those happy dispositions where it is almost superfluous. ” (Gibbon)

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Posted : April 20, 2017 8:36 am
Larry Best
(@larry-best)
250+ posts Registered

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

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Posted : April 20, 2017 8:47 am
GeeOddMike
(@geeoddmike)
500+ posts Registered

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!

DMM

The power of instruction is seldom of much efficacy except in those happy dispositions where it is almost superfluous. ” (Gibbon)

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Posted : April 20, 2017 8:48 am
GeeOddMike
(@geeoddmike)
500+ posts Registered

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

DMM

The power of instruction is seldom of much efficacy except in those happy dispositions where it is almost superfluous. ” (Gibbon)

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Posted : April 20, 2017 8:55 am
Larry Best
(@larry-best)
250+ posts Registered

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

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?

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Posted : April 20, 2017 10:15 am
base9geodesy
(@base9geodesy)
100+ posts Registered

In USVI OPUS returns heights that are related to VIVD09.

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Posted : April 20, 2017 12:09 pm
Larry Best
(@larry-best)
250+ posts Registered

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.

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Posted : April 20, 2017 12:52 pm
Larry Best
(@larry-best)
250+ posts Registered

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

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Posted : April 20, 2017 12:58 pm
SPMPLS
(@spmpls)
100+ posts Registered

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.

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Posted : April 20, 2017 1:02 pm
SPMPLS
(@spmpls)
100+ posts Registered

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.

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Posted : April 20, 2017 1:07 pm
Larry Best
(@larry-best)
250+ posts Registered

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.

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Posted : April 20, 2017 1:17 pm
GeeOddMike
(@geeoddmike)
500+ posts Registered

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.

DMM

The power of instruction is seldom of much efficacy except in those happy dispositions where it is almost superfluous. ” (Gibbon)

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Posted : April 20, 2017 1:23 pm
SPMPLS
(@spmpls)
100+ posts Registered

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?

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Posted : April 20, 2017 1:34 pm
Larry Best
(@larry-best)
250+ posts Registered

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.

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Posted : April 20, 2017 1:49 pm
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