Anyone else seeing this:
Have a FIRM map with an AE zone. I was sent a plot of contour map from another surveyor based on NAVD88 elevations with a firm plotted on it, they appear to be 4' higher than the FIRM map
Two things to check. Datum of the FIRM and reference benchmark for the topo...
both NAVD88
the topo checked bench marks with OPUS about .25' difference
My next question would be what elevations are you looking at on the FIRM panel? If they are BFE lines 4 feet is not unusual. Of course you still need to check the FIS profile...
Guess what I'm saying is at the edge of the flood plain the ground is 4' higher than the BFE, in otherwords, no houses will show as in the flood plain, everything will be out when surveyed cause the flood plain needs to narrow so far it's almost to the bank of the river.
It appears to be a datum shift, the FIRM map says NAVD88 and the surveys are NAVD88, just wondered if anyone else is seeing this. The old flood plains hit the NGVD29 ground elevations pretty well.
If you have old and new flood study profiles compare the 2. If the datum is shown as different but the numbers are nominally the same you have your answer. A Call to a map specialist may reveal they already know.
> It appears to be a datum shift, the FIRM map says
Grammar Police, please pull over, sir. FIRM is an acronym for Flood Insurance Rate Map, so saying "FIRM map" is like saying "Flood Insurance Rate Map map." (The equivalent sin in the car world is to say "VIN number"; in the banking world it's "PIN number.") thebionicman got it right with "FIRM panel."
I'm letting you off with a warning this time, sir, but please be more careful in the future. (Kick starts police motorcycle, roars off into the flow of traffic.)
look in your rear view...he just gave you the finger.:-)
From now on I'm calling it a FIR Map. 😛
It's not unusual to find land shown to be inside a flood zone, but when you survey it, it's higher than the BFE. Is it possible that fill was placed? Double check everything, then double check everything again, and if you still feel the land is higher than BFE, then have the owners apply for a LOMA.
FIRM -worse than I first thought
The panels appear to be worse that we first thought, three different surveying and engineering companies are seeing the same issue. The panels' BFE's are 4.5 to 5 feet low.
That, of course is good for the people close to the flood plain (actually using the elevations there is no flood plain except right at the river), but I'm not going to certify anything using those panels, what a mess!!!! 🙁
Only thing I will do is use the old panels from the 1960's, they make sense!!! 🙂
Be very careful with FIRM data!!!!
FIRM -worse than I first thought
Have you checked the FIS profiles? Differences of feet are not unusual. That's why FEMA policy is the profiles rule...
FIRM -worse than I first thought
I haven't yet, been told another engineer has and sees the same issue with them, but the BFE's are so far out, not sure how that will change anything.
The profiles and the BFE's on the older FIRM matches seamlessly.
This is the first project we've done using the new FIRM, I don't think it's our problem to fix the mess.
But it looks like the client will have to get his own flood study done over the top of the FIRM, if he wants to proceed with his project.
FIRM -worse than I first thought
Looking at the profiles; they match the BFE's, so it has the same issue.
Talking to a surveyor/engineer, he saw it also and had to do a flood plain study for the small subdivision he was working on. That was 1/2 a mile upstream from ours, looks like anything on this map will need to be redone, not much option for anyone.