Since the historic flooding in Baton Rouge, LA area two months ago, many people are having to get Elevation Certificates. I have several clients that had old ones but were told they needed an updated one so they called me. The issue I've encountered is the comparison between the old elevations with my new data. It seems that my elevations are typically a foot or so lower than these old ones. And when I say old, the ages of the previous ECs range from the 1980s to early 2000s. I'm using a 1-hr static session to get my elevations, as well as pre- and post-static VRS shots as a check. I've checked-in on newly installed benchmarks in my area that were established as part of the height modernization study, and my elevations are usually within 0.1'. I can explain a 0.1' or so with the datum change, but where is a foot coming from? I'm fairly sure these old ECs were leveled from existing benchmarks, so the difference must be coming from the old benchmarks themselves, correct?
As you all know, 1 foot lower in Zone A/AE is $$$$ in insurance premiums. I'm not going to say their old one was wrong, because it was probably right at the time. But mine is right too. And I know their house didn't settle a foot either. Can you guys help me explain the difference to my clients in terms they can understand?
Subsidence? There are benchmarks in some areas of California that are now several feet lower than the published elevation. Your area is known to be subsidence prone, so if a surveyor just grabbed a bench mark and used the published elevation without making any effort to verify the validity the elevation relative to the datum at the time of their survey, well, good luck with that....
I'll buy subsidence if the benchmark used was several decades old. Typically subsidence is more of an issue in areas further south, like New Orleans. But if the benchmark was old enough I suppose it could subside a foot.
W
andrewm, post: 395730, member: 10888 wrote: Since the historic flooding in Baton Rouge, LA area two months ago, many people are having to get Elevation Certificates. I have several clients that had old ones but were told they needed an updated one so they called me. The issue I've encountered is the comparison between the old elevations with my new data. It seems that my elevations are typically a foot or so lower than these old ones. And when I say old, the ages of the previous ECs range from the 1980s to early 2000s. I'm using a 1-hr static session to get my elevations, as well as pre- and post-static VRS shots as a check. I've checked-in on newly installed benchmarks in my area that were established as part of the height modernization study, and my elevations are usually within 0.1'. I can explain a 0.1' or so with the datum change, but where is a foot coming from? I'm fairly sure these old ECs were leveled from existing benchmarks, so the difference must be coming from the old benchmarks themselves, correct?
As you all know, 1 foot lower in Zone A/AE is $$$$ in insurance premiums. I'm not going to say their old one was wrong, because it was probably right at the time. But mine is right too. And I know their house didn't settle a foot either. Can you guys help me explain the difference to my clients in terms they can understand?
Sounds like a difference in datum's. What year was the last study done?
Do you have the old EC's? You could check what BM's were used and what the date of the published elevation was relative to when they were actually set and originally determined. Or you could occupy one and get an OPUS solution, then compare to what was used for the EC. We have some areas in California that are subsiding at a foot or more year at the moment and it is very localized.
Mike Lacey, post: 395735, member: 303 wrote: W
Sounds like a difference in datum's. What year was the last study done?
I did a datum conversion from NGVD29 to NAVD88 and the difference was ~-0.1'
SPMPLS, post: 395736, member: 11785 wrote: Do you have the old EC's? You could check what BM's were used and what the date of the published elevation was relative to when they were actually set and originally determined. Or you could occupy one and get an OPUS solution, then compare to what was used for the EC. We have some areas in California that are subsiding at a foot or more year at the moment and it is very localized.
I have some of them. I tried that for one that had a NGS BM. It wasn't listed on NGS anymore but I was able to dig up an old data sheet. When I tried to find it, it had been destroyed by new construction. Some of the ECs have BMs listed that were installed by the surveyor, so I have no way to get that data sheet.
Well, based on my own experiences here and limited knowledge of the subsidence issues in your region, I strongly suspect it to be a contributing factor, especially if your work is checking closely with a recent height modernization survey. If you had some idea of the approximate date the now destroyed NGS BM was set, you could possibly find another of the same vintage in the area that is still intact and observe it to see what you get relative to the published elevation. If some pre-existing NGS marks were incorporated into the HM network, you could pull the datasheets and look at the superseded positions to see if they have been moving downward and by how much.
Just my 2 cents from the peanut gallery.
andrewm, post: 395730, member: 10888 wrote: Since the historic flooding in Baton Rouge, LA area two months ago, many people are having to get Elevation Certificates. I have several clients that had old ones but were told they needed an updated one so they called me. The issue I've encountered is the comparison between the old elevations with my new data. It seems that my elevations are typically a foot or so lower than these old ones. And when I say old, the ages of the previous ECs range from the 1980s to early 2000s. I'm using a 1-hr static session to get my elevations, as well as pre- and post-static VRS shots as a check. I've checked-in on newly installed benchmarks in my area that were established as part of the height modernization study, and my elevations are usually within 0.1'. I can explain a 0.1' or so with the datum change, but where is a foot coming from? I'm fairly sure these old ECs were leveled from existing benchmarks, so the difference must be coming from the old benchmarks themselves, correct?
As you all know, 1 foot lower in Zone A/AE is $$$$ in insurance premiums. I'm not going to say their old one was wrong, because it was probably right at the time. But mine is right too. And I know their house didn't settle a foot either. Can you guys help me explain the difference to my clients in terms they can understand?
What does your local flood plain administrator say?
andrewm, post: 395739, member: 10888 wrote: I have some of them. I tried that for one that had a NGS BM. It wasn't listed on NGS anymore but I was able to dig up an old data sheet. When I tried to find it, it had been destroyed by new construction. Some of the ECs have BMs listed that were installed by the surveyor, so I have no way to get that data sheet.
What was the name of NGS benchmark that was used? Do you know the PID? Destroyed benchmark datasheets are still available if you know where to look.
Joshua Kent at LSU is someone I would contact as part of my investigation.
http://www.c4g.lsu.edu/index.php/contact-us/4-uncategorised/4-joshua-kent-ph-d
Jim in AZ, post: 395741, member: 249 wrote: What does your local flood plain administrator say?
I haven't called them yet, but based on how they handled the post-flood permitting and substantial damage assessment process, I doubt they'll be much help. They were totally clueless on the whole process. But I'll give it a shot.
Bow Tie Surveyor, post: 395742, member: 6939 wrote: What was the name of NGS benchmark that was used? Do you know the PID? Destroyed benchmark datasheets are still available if you know where to look.
I found the datasheet, but the BM was destroyed.
SPMPLS, post: 395746, member: 11785 wrote: Joshua Kent at LSU is someone I would contact as part of my investigation.
http://www.c4g.lsu.edu/index.php/contact-us/4-uncategorised/4-joshua-kent-ph-d
I have known Josh very well for many years. I'm sure he'll say subsidence also.
And that's certainly a much easier explanation to my clients without having to say the old one is wrong, because it may not be.
Pray you're never required to be in IS Net World. Holy hell what a nightmare to get signed up in that!
It could be a Geoid issue.
andrewm, post: 395756, member: 10888 wrote: Pray you're never required to be in IS Net World. Holy hell what a nightmare to get signed up in that!
Wrong thread...
NGVD 29 vs NAVD 88.......are your certain about that difference of 0.1'? I know if is different based on your location, but in my state the smallest difference I have ever seen was 0.70' and the largest was about 1.40'. Can you pull a nearby benchmark datasheet that displays the value in each datum as a check?
andrewm, post: 395750, member: 10888 wrote: I haven't called them yet, but based on how they handled the post-flood permitting and substantial damage assessment process, I doubt they'll be much help. They were totally clueless on the whole process. But I'll give it a shot.
You should contact FEMA and let them know that your FPA needs updating...
The old flood plain maps were based on reference monuments that were set perhaps in the 1930's my whatever method was used. Those reference monuments were golden for the old flood plain maps. However, their listed elevation and their actual 29 elevation may have never agreed but the maps were based on the listed elevation. That is part of why I issued the warning a month or two ago about being careful about GPS-derived elevations. The new elevation is probably correct and the adjustment to what the old 29 elevation should have been may be correct but that may have no relation to the listed elevations of the reference monuments used on a specific flood plain map built from those reference monuments.
