The error analysis done by OPUS is always in the .000 range.
I get OPUS-S orthometric elevations with error estimates in the 0.06m range. Ellipsoid elevation errors are in the millimeter range. That's the geoid model.?ÿ
you're stuck with the bench marks shown on the FIRM, if they show them as control.
I'm skeptical that the bench marks have much of anything to do with the data they used unless I was involved with it.
There is so much that goes into the BFE determination, you have to know the sequence of events.
How were the panels for the photos controlled if they did a community map, how were the sections collected for the valley cross-sections. It all counts.
I would really hesitate to dismiss a bench mark that's on the FIRM map. You say it's lower than CORS so it results in the structure being below BFE and if you use CORS you're above.
It's meaningless if CORS wasn't the basis of the contour data and you're taking on that liability. Was CORS even there when the mapping was done?
There wasn't a CORS point within 150miles when our local mapping was flown for FEMA control.?ÿ
What purpose would an Ortho have in establishing a NGS benchmark?
Well, considering the NGS benchmarks reference orthometric elevations, I'd say a lot.
The benchmarks were established years before Orthos were developed.
I don't understand this statement. Geodetic levelling campaigns were (and still are) used to establish ortho elevations for NGS benchmarks.
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The geoid model has something to do with it I believe. That's another statistical use of a computer to come up with finite answers.
How do you think they got final orthometric elevations from the original NAVD88 levelling campaigns, if not with a computer and utilizing statistics?
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For what it's worth, GEOID18 computations place the 95% confidence error at +/- 4cm in the Craig area:
L 435 (SS1269) near Craig, MT. Cooperative Base Network Station/BM...occupied by OPUS twice in 2021...
Published NAVD88: 1054.011 m (leveled first order)
Published Ellip H: 1040.669 m (network accuracy 2.29 cm)
GEOID18: -13.342 m
Ellip H-GEOID18: 1054.011 m?ÿ
OPUS1 NAVD88: 1054.000 m
OPUS2: NAVD88: 1053.993 m
Seems like everything agrees very well. Disk in rock outcrop...
@mightymoe There are no benchmarks referenced on the newer 2013 era FIRM maps or anywhere else in the FEMA documentation. The older NAVD29 maps did have benchmarks designated on them that were done by private contractors, not NGS.
I won't get into how FIRMS and BFE's were developed. That's another different subject.
My question is how do you relate the Elevations I find on the ground to the Elevations on a FEMA FIRM map most accurately.
@rover83 Statistics! I think statistics should be used to evaluate the accuracy of a benchmark, not establish it.
I misunderstood your use of Ortho. I thought you were referring to orthophotography.
Your replies indicate you're a bit flustered, but it also seems like you're not seeing the forest for the trees.
The answer to your question is to run conventional levels from original benchmarks, just like they did originally.
You may not like that answer, but it doesn't matter what you think of: who did what, how well they did it, whether you like what they did, or a particular methodology, etc.
You came here asking for help because a government official whose job it is to understand these things disagrees with you.
The peanut gallery has consistently opined that your "discrepancies" are unusual for the normal quality of effort seen during this course of work, and are potentially blunders, which may coincide with the gov't official.
It may not be possible to use original methods, or benchmarks, to establish orthometric elevations with respect to the correct vertical datum, but it is incumbent upon you to know how to use your technology to get the right answer, else you're going to have a tough row to hoe.
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The OP indicates that he is unaware of tools to evaluate the heights of benchmarks published by NGS. Retrieving detailed information about these points is readily using tools available on the following pages:?ÿ
1.?ÿ https://geodesy.noaa.gov/datasheets/ ?ÿ - web page to retrieve information for points in the NGS data base. As indicated in other replies, first check the source of the published height. Examples of entries are shown here:
2.?ÿ https://beta.ngs.noaa.gov/datasheets/leveling-projects/index.html ?ÿ- beta site page to display information about a level project using it's unique identifier.?ÿ
3.?ÿ https://geodesy.noaa.gov/cgi-bin/phase1.prl ?ÿ - input page for the retrieval of field adjusted heights for a level project. It allows selection of corrections to be applied. Requires an input file. The input file must be a plain text file containing only the level project identifier e.g. L12345.
An output file is created containing unadjusted field observed heights and section lengths. Essentially it shows field observations with chosen corrections applied and the heights only propagated from one starting BM. These are NOT final adjusted heights! ?ÿ?ÿ
4.?ÿ https://beta.ngs.noaa.gov/GEOID/xGEOID20/computation.shtml ?ÿ- interactive computation tool prompting for latitude, longitude, ellipsoid height, output epoch (optional) and name
The input data listed above is used to invoke the NGS hybrid and gravimetric geoid models providing heights and statistics for the entered points.
Finally.?ÿ
The NGS data sheet clearly identifies the datum associated with the displayed value in both the ??Current? and ??Superseded? sections.?ÿ
The continental bias and tilt between NAVD88 and our current understanding of the geopotential is being addressed with the new vertical datum.
And even more finally??
As mentioned elsewhere, we need to consider whether a physical monument has been subject to some displacement/disturbance.
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The instructions for the EC are pretty simple, reference a published BM and use the same datum from which the FIRM BFE was established.?ÿ Like @olemanriver stated above, my differences between my OPUS observations and the published BM elevations have always been comparable, never more than 0.12' difference between the two.?ÿ Without knowing more, I'd have to use the NGS BM as being what I hang my hat on.?ÿ
From a liability point of view, I think I'd rather have a contractor add 0.1' of fill to the site prior to construction instead of having multiple buildings constructed just to find out later that they were built with a finished floor 0.1' or more below a BFE.
@kevin-hines That's ridiculous. Floodplain boundaries and Base Flood Elevations we're not created by means to develop elevations to a 0.1 of a foot.
There are no references that FIRM elevations were established using any NGS Benchmarks. I got a letter from FEMA regarding NGS Benchmarks. FEMA stated " We we're not involved in creating, maintaining, or certifying the accuracy of NGS Benchmarks". All I had asked for was a yes or no answer letter regarding "Are OPUS derived elevations acceptable for use on FEMA Elevation Certificate's."
FEMA does not say which datum or how you have to derive your elevation. It gives you the professional the option to decide. You can use ngvd29 navd88 or navd 88 and geoid model used . The way i read the certificate is i just need to check the box or write in what i used. I think they would prefer the latest navd 88 . ?ÿBut geezers it took them years to even start the surveying campaign across the usa on tributary??s to move from ngvd29 to navd88. And if all comes together we will all be working on a completely different vertical datum in the near future and a datum thats not originally established via leveling. Should be interesting in the future. Lots to learn and be ready to improvise on for sure.?ÿ
It would be chaos to use the new elevations to control existing FEMA maps.
Nothing would fit, elevations in my area would be 2' off. You have to use NAVD88 control that created the maps.?ÿ
There will need to be new FEMA maps issued, which is an engineering and political process.?ÿ
It will be decades before 2024 elevations will be available for a new generation of FEMA maps. I would guess not before 2035 in many areas, maybe much longer than that.
Locally 88 FEMA maps here were first voted on in 2010 if I remember correctly and then a few more years before they came on-line.
The flights for the contouring happened in 2003.
I agree and would tend to use local benchmarks and not OPUS. ?ÿ
BFEs are really just a depth above the bottom of the waterway (except in the case of tidal influence or a backwater). So whatever control was used for the cross sections is the control which is best for determining the BFE height at any given location.
