NGS Benchmarks versus OPUS derived Elevations
I'm currently in a dispute with a Community Flood Plain Coordinator over the basis of determining elevations for FEMA Elevation Certificate's. I regularly observe one foot differences in OPUS derived elevations when checking in to NGS monuments with published elevations. I can see what elements are involved with OPUS derived elevations, but not NGS monument elevations. Therefore I tend to favor the OPUS Elevations.
The dispute with the Community Official came about because the installation of a sewer system nearby has elevations derived from a NGS benchmark and my OPUS derived elevations (two separate observations which differed .003 meters). The OPUS derived elevations result have the building site over a foot above the BFE. The NGS monument result are the building site is 0.1 foot below the BFE.
I realize this difference in distance should be considered insignificant, but try explaining that to a County official. I'm interested in others opinions.
Interesting situation. What happens when all the actual monuments are gone and not getting replaced? Can you share where you are/what monuments you are comparing to? I wonder if others have similar results in your area.
When you say 1 foot from OPUS vs a NGS Benchmark. The first thing that comes to mind is NAVD88 vs NGVD29. Roughly. When I compare OPUS to a NGS Benchmark I don’t see that much difference unless 1. Disturbed monument 2. Multi path environment 3. I chose the wrong datum. But I am on the east coast but have seen about the same in GA TN AL over the last many years. NGS Datasheet and OPUS are usually on same datum unless you have some older data sheets that still have ngvd29 on them. Ow I do realize that OPUS is Geoid derived NAVD88 and will not match exactly depending on Geoid quality in your area and such to the true benchmarks. 1 foot is a large separation. Is that a common occurrence in your area.
In my opinion, you are the professional signing the certificate. If you are comfortable signing the cert based on the OPUS elevations, so be it. In the end your signing the certificate puts you on the hot seat if something is not correct.
In general I would say hold the NGS benchmarks, but I don't know where you are, the status of the bench, and all that goes with analyzing benchmark data.
The community official in my area is an architect, so I feel your pain when it comes to interpreting flood elevation certificates.
I'm just guessing the floodplain information was developed using NGS benchmarks. So if I wanted to provide relevant elevations to the floodplain it would make sense to compare apples to apples. If the floodplain information was based on OPUS observations then I would use OPUS. You have obviously shown that NGS and OPUS do not match. So using 2 different elevation realizations doesn't seem wise.
@joe-the-surveyor That is my first hunch as well. I just did a certificate and the last time it was done and particular county still uses ngvd29. So about a foot. I checked to a few county monuments and a ngs benchmark. I was using network rtk. Ngs benchmark was .075 ft. And ngvd29 county monuments were 1.21 and 1.18. Using the ngs tools i did the transformation as a check and was whithen the conversion tolerances of rtk and ngs nabd 88 to ngvd29. No statement of accuracy on the county mons. I placed navd 88 on certificate. Boss ls signed county said we need it on ngvd29 so he took the conversion and changed it. No big deal just have to have the metadata match the values etc. i use to know the rough difference from florida to Mississippi on that as i had to do support in those areas. Somewhere in the.5 ft to 1.6 ft depending on where someone was. Now that was a while back so my numbers might be wrong. Just using my memory.
I can see what elements are involved with OPUS derived elevations, but not NGS monument elevations.
The NGS is pretty good about putting the source of the orthos on the datasheet. It'll say how it was determined and when it was adjusted.
It's also pretty clear whether an ortho is NAVD88 or NGVD29, and if there are superseded value(s) they'll be on there as well. That can provide additional clues.
If an NAVD88 NGS benchmark (levelled and adjusted by the NGS per the datasheet) isn't matching OPUS-derived orthos using the latest geoid, then I would first be looking at whether there is a good published geodetic position (LLh) for that mark. If not, there's a good chance that the mark was not utilized in the latest hybrid geoid model and there could in fact be a discrepancy between what the geoid model says the separation is and what you are measuring ellipsoid vs the published ortho.
But one foot is a big difference. I don't know of any places in CONUS where the estimated uncertainty of the geoid is that large. Not that there couldn't be, I just haven't seen any as of yet.
So there could also be local movement/subsidence affecting the mark as well.
@olemanriver There is about 3 foot difference in NGVD 29 and NAVD88.
I have found two NGS monuments to differ by over 4 feet between the monument versus their published Elevation. This has been the most extreme difference I have observed. In that instance the monument suspiciously is on a 8 inch pipe 4 feet high.
@skeeter1996 where are you located. Geographically…. When you say between two monuments you see a difference not datum difference. That shows a disturbed monument of one or both assuming good measurement techniques were used. 3 feet between ngvd29 and navd88. That is a difference in datums. Opus like above was stated is no where in conus using a geoid is that far off. So I guess whatever monuments you are holding are disturbed or subsidence etc. No matter which way you go having that information showing the difference in published vs opus you can easily show the county that your opus results are more realistic than the published marks value. And you can convert to ngvd29 from navd88 opus derived point. If i were licensed and i am not. I would have a nice 1 to 2 page little graphic and or report showing that the monuments on the ground do not check themselves and your opus results from two different sessions are better. You have proved movement on the ngs monuments if I understand correctly. So they are bad bad bad lol 😂.
Not all NGS datasheeted elevations are created equal. Are we talking about benchmarks with levelled elevations?
Also, are we talking about redundant 2+ hour OPUS-S solutions? ie/ have you eliminated the possibility of a measure up blunder? If so I'd have more confidence in the OPUS results.
It is possible that the geoid modelling in the area is flawed.
@rover83 What purpose would an Ortho have in establishing a NGS benchmark?
The benchmarks were established years before Orthos were developed. The benchmarks to my knowledge were established before any of that technology was developed. Most benchmark haven't haven't been located since they were set and none have had their ave had their elevations verified
@norman-oklahoma I have more trust in the OPUS Elevations. Redundant measurements have always matched within .003 meters. The error analysis done by OPUS is always in the .000 range. It's a fixed height tripod. Any measure up error would be less than .03 feet.