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NGS Benchmarks versus OPUS derived Elevations
Skeeter1996 replied 1 year, 8 months ago 21 Members · 54 Replies
You use control that created them.
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.
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:
1. https://geodesy.noaa.gov/datasheets/ – 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. https://beta.ngs.noaa.gov/datasheets/leveling-projects/index.html - beta site page to display information about a level project using it’s unique identifier.
3. https://geodesy.noaa.gov/cgi-bin/phase1.prl – 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!
4. https://beta.ngs.noaa.gov/GEOID/xGEOID20/computation.shtml - 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.
The NGS data sheet clearly identifies the datum associated with the displayed value in both the ??Current? and ??Superseded? sections.
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.
The instructions for the EC are pretty simple, reference a published BM and use the same datum from which the FIRM BFE was established. 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. Without knowing more, I’d have to use the NGS BM as being what I hang my hat on.
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 . But 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.
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.
There will need to be new FEMA maps issued, which is an engineering and political process.
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.
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.
@skeeter1996 I said nothing of the FIRM elevations being established by using NGS benchmarks, but if you read the attached screen shot of a FIRM panel, one could make that argument.
I believe I said, “reference a published BM and use the same datum from which the FIRM BFE was established”. The line immediately prior to item C2. a) of FEMA Form 086-0-33(12/19) states, “Datum used for building elevations must be the same as that used for the BFE”.
To answer your question, YES, you can use OPUS derived elevations. The instructions for this are:
Just because it is an option, that doesn’t guarantee that the approving authority will accept your methods.
I gave my opinion on how to keep your liability to a minimum. If you are continuously have 1′ difference from your OPUS solutions and published benchmark elevations you might be able to argue that your procedure is more accurate than those used used to establish the published benchmarks. Then again, one could argue that since you continuously have a 1′ difference, there may be something wrong with your data processing procedures. Good luck with whatever you decide.
If this site is in Craig, Montana (as you mentioned), then clearly you are not looking at anything FEMA:
FEMA FIRM Map Number: 30049C1425E (9/19/2012); Community: 300038 (Lewis & Clark County, Montana)
Has this nice little blurb in the lower left corner of the map:
And also clearly shows six (6) NGS benchmarks that have current NAVD88 & legacy NGVD29 orthometric heights (1st Order; 2nd Class) from adjusted leveling in 1991:
SS0852 (W 324): https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=SS0852
SS0849 (V 324): https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=SS0849
SS0848 (W 9): https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=SS0848
SS0845 (Q324) : https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=SS0845
SS0844 (V 9): https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=SS0844
SS0843 (U 324): https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=SS0843
If that is indeed the case, then you, sir (ma’am), are a charlatan…
@kevin-hines It’s not always 1 foot. Sometimes 0.1 foot, once 4.9 foot. I basically know where the data comes from to get a OPUS derived elevation. Not so for a NGS benchmark. Most of the ones I’ve measured are of the 1960 vintage. I’m relatively sure NGS has not physically resurveyed them to update the elevation from NAVD1927 to NAVD1988. They just ran them through a computer program. Technology and Knowledge about the Geoid has changed significantly since the 1960’s. The Geoid is just geodists’ best guess of the earth’s gravitational and rotation that best fits precise elevation calculation. It’s being refined all the time. I’ve not seen any data that shows that NGS benchmarks used any kind of a geoid model.
I’ve attended many continuing education seminars. Some were given by NGS representatives, some by FEMA representatives. The message I got from the NGS representatives was, “It doesn’t get any better than OPUS”. NGS is promoting GPS on Benchmarks. That makes me suspicious of the NGS Benchmarks.
I’m leaning towards only using OPUS for FEMA Elevations. I’ve been somewhat surprised of the support NGS Benchmarks are getting. Most every Surveyor in my area is verifying the NGS Benchmarks with an OPUS observation.
@kevin-hines Usually construction plans have benchmark designated as the control to be used for elevations. I would be pretty darn hesitant to use OPUS, NGS Benchmark or other vertical control that was not shown on the plans.
You, being the expert measurer that you are, must be correct. Do it your way, I DON”T CARE! You asked for advise, I gave it, and because you don’t agree with an opinion that isn’t the same as yours, you want to engage in an on-line argument. Since your profile does not indicate if you are licensed or not, I will advise you in this manner. If you are licensed, complete sign, & seal the form. If you are not licensed, ask your boss how he/she wants it completed. Like I have previously mentioned, just because you have an optional way of establishing an elevation, does not guarantee that the approving authority will accept your methods.
@skeeter1996 well I think opus is fine. It has a tie to navd88 any way. Only time I would not use GPS or OPUS on a elevation certificate is when its to close for geoid and gps accuracy for a client to have to pay flood insurance. Then i would rune levels if possible. The reality is though that many marks have not been re leveled and not all leveled marks have had GPS observations on them. That is a reason for GPS on Benchmarks. With the airborne gravity and studies i have seen the passive marks will not be as important in the new datums. Not because they are inferior but cost to maintain them is to high. I love visiting old marks and many are very stable. And have served as a great source for many years many many years. If i were rich beyond my means i would be visiting and keeping them updated as a way to preserve and keep them. But i am not and i wish somehow the local states and or surveyors could preserve them. It is just the federal government is not.
The new datum will i am sure have its obstacles but will be designed to possibly eliminate passive marks all together as far as maintaining them. Opus is a tool by NGS and a benchmark that has been published is access to a datum. Opus is access to the same datums as well. Just not through that physical monument. It is through indirectly many many monuments over time. They both have pros and cons.
@michigan-left I’m puzzled by your post NGS Benchmarks were converted from NAVD29 to NAVD88 by adding 3.7. The only time I’ve converted NAVD29 benchmarks to NAVD88, I’ve used NGS’s conversion program. It varies sometimes considerably depending on how close you are to the mountains and what your Latitude, Longitude is.
FIRM map accuracy is another subject.
I stand corrected. The FIRM Panels do have NGS Benchmarks shown on them. I usually only retrieve FIRMETTEs from FEMA to locate the nearest cross section to determine the location in the FEMA Flood Insurance Study. FIRM map quality is another subject. The subject is OPUS versus NGS Benchmarks.
How old are you? What’s a charlatan. That sounds feminist.
@skeeter1996 it sounds Shakespearean to me. We go to unlevel the datum charlatan!
@dave-karoly Yeah I think you’re right. Is he trying to intimidate me with his superior culturedness?
Let’s just say I’m old enough to know that you’re likely in more trouble than you’re probably aware of, and it’s likely that you will be the last to know.
You might benefit from the collective wisdom at this site.
I can be a salty SOB, and I apologize for that, but everyone here is trying to help you.
Thanks for identifying benchmarks in the area.
Note that the first two points (SS0852 and SS0849) have NAVD88 adjusted heights. Their NGVD29 heights are both indicated to be ??ADJ UNCH? which means: Manually entered unverified output from a least squares adjustment of level data.? I would have low confidence is ADJ UNCH heights. Nonetheless subtracting the NAVD88 and NGVD heights for the two PIDs shown yields differences of 4.07 and 3.09 (in units of feet).
I ran these points through some of the tools shown in my first post on this topic. With only my iPad Pro on hand I am unable to run Phase1.
Using the Project Identifier from PID, I find that the two PIDs were ONLY used in one NGS level project: L18721.
Running the Leveling Project tool yields the following information.
From L to R
Project summary and statistics, actual section leveling data (uncorrected and unadjusted), graphics showing location of SS0849, the entire level project and SS0852.
The OP-er should read https://www.ngs.noaa.gov/PUBS_LIB/NAVD88/navd88report.htm for an better understanding of the shift to NAVD88 from NGVD29.
Note that the ??List Observations? option shown in the graphic shows section distances as well as height differences. It also shows both forward and backward runs as well as reobservations.
When I return, I will run the Phase1 tool. It??s on my checklist??
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