I have to do the construction layout for a high-rise mixed use project on an ocean front property. We have plans which included a previous ALTA survey. The first thing we did was set up our Trimble R8 to get an elevation check on some MH rims and curb grades. We found a consistent discrepancy of 0.50' low throughout. I then asked a friend of mine to check me with his R10 GNSS, and he agreed with me by 0.02'. Just to be sure, we borrowed another friends static unit, did a 3 hour occupation, and sent the data to OPUS. Same result, 0.02' agreement with me. The original surveyor used published benchmarks that were in NGVD29, and I don't dispute his work using those benchmarks. My problem is the property is in the FEMA VE zone, and a building the place 0.50' low could be a huge liability issue. Any thoughts?
Do the panels show any marks? Nothing beats using 2 marks used in determining the flood hazard.
Panel marks are long gone. We did check the published benchmarks, one 30 years old the other 60, and agree that we disagree. The point is, over time government set benchmarks will go the way of the dinosaurs. All we will have is GPS/GNSS and the buildings will still be too low. We have been told to hold the plans and the associated elevations. What would you do?
Did you shoot in any USGS or City or County benchmarks? Also the building would be built 0.50' higher if that benchmark is really 0.50' low.
The published benchmarks are county. I reversed it, I'm low the plan is high. The building would be 0.50' low.
I am assuming you are missing by 0.5' after converting to NGVD 29?
Is the FIRM in NGVD 29 or has it been updated to 88?
It seems like you don't really have much of a choice but to rely on the plans, which were most likely derived from the original ALTA survey you mentioned that apparently had elevations on it. You must tie into said plan and layout using the site datum. UNLESS it specifically references another datum or something in the construction drawings.
If it's a high-rise, muilti-use building it has to have gone through review and approvals at this point.
NGVD 1929 vs NAVD 1988 ?
I know I dealt with this working along the Mississippi River. I had done an ALTA and used a city benchmark. I ended up finding two of the benchmarks from the flood map and verfied they were good but it took a lot of work to do this. It was a similar situation, the building was either in the flood zone by a few tenths of a foot or just outside of it by a few tenths.
I also know in my area OPUS solutions won't agree with known benchmarks by a few tenths of a foot.
Its always a little questionable what datum they used when the original flood maps were prepared. I would maybe check with any other surveyors you know that might shed more light on it.
techls, post: 379307, member: 6448 wrote: The published benchmarks are county. I reversed it, I'm low the plan is high. The building would be 0.50' low.
FEMA should state their elevation system, here it was the bench system in 29 and the new 88 ones are also based on the bench marks, GPS bounces around in my area, up and down with each new Geoid model and Epoch.
You HAVE to use the site plans unless given permission by the owner to change, I would call to the attention of the owner what you have discovered and let him decide what to do.
But first I would FIND OUT FOR SURE what the FEMA maps are based on, that shouldn't be difficult at all.
techls, post: 379304, member: 6448 wrote: We have been told to hold the plans and the associated elevations. What would you do?
I would document everything, and hold the benchmarks as told. BUT...the issue will come up when and if you are asked to do a FEMA cert or to certify to a certain datum. Then, the question is what value do you hold.
This should be clearly stated in the communications to the stakeholders.
dmyhill, post: 379314, member: 1137 wrote: I would document everything, and hold the benchmarks as told. BUT...the issue will come up when and if you are asked to do a FEMA cert or to certify to a certain datum. Then, the question is what value do you hold.
This should be clearly stated in the communications to the stakeholders.
This should be clearly stated in the WRITTEN communications to the stakeholders.
another possibility: could someone have used the wrong antenna definition, or used an ARP to phase center offset that is close to 0.5' ?
Moe Shetty, post: 379319, member: 138 wrote: another possibility: could someone have used the wrong antenna definition, or used an ARP to phase center offset that is close to 0.5' ?
I've been bitten by this before. It made a difference in elevation of almost 0.30' too low if I am not mistaken, but is has been a couple of years ago.
FIRM maps are in NAVD88 in this county. I may be making too big of a deal out of it, being in a VE zone. It will be hit with waves, not gradual flooding when the "big one" hits, so 0.5' won't make much of a difference.
Building the unit with the lowest horizontal structure beam below the BFE will be a big mistake for you and all involved. The county is required by FEMA to make sure that no structures are allowed to be constructed below the BFE. The source of the error is not the point now, you are now the Registered Surveyor responsible so either fix the problem are walk away from the project. Not the owner or the engineer or anyone else can give you "Permission" to build below BFE, if needed get the local Flood Plain Administrator involved to determine the correct BFE and datum at this site.
Before anything is built, this needs to be solved.You can't build the structure based on what others tell you to do.
I would check with local officials to see if the building is constructed based on your numbers would cause any issues. Better to be to high in a flood zone than to low.
Did they really design it that close? Are you saying 0.5' will make a difference whether or not the lowest horizontal structural member is below the BFE? Around here you usually wouldn't get approval without being probably 2 ft above minimum.
I can't imagine that's the case.....I'd probably mention this to the owner and designer and make sure they've considered it, but i still say you have to work off the approved site plan and use that project datum.
Sit down with the community floodplain administration and review the CLOMR-F and the Floodplain Development Permit...errr...there are the proper permits in place??
from my experience I would not use OPUS, CORS or a VRS to stake elevations based on flood plains.
I haven't ever seen one based on that data, maybe there are, but I haven't seen it.
And if I did, I would need to know the EPOCH, Geoid, ect that was used.
It does little good try to merge data from 2000 based on the 2012geoid.
When it comes to staking a project it needs to be an apples to apples data application.
How are you converting the 29 elevations on the published benchmarks to 88? A conversion program? Or perhaps there are published 88 values using an outdated geoid model? I know around here the variation can be at least 0.7' between geoid 96 and 12a. I think your project being oceanfront, there could be a big difference between geoid models. Vdatum can be used to see what the differences are in your area.