I have a case where a FEMA reference mark referenced by FEMA varies from NAVD88 by about 0.3 feet. I checked it against other FEMA reference marks and found them to vary between each other by about 0.2 feet, some higher, and some lower. I checked the NAVD88 USGS monuments, and they are all correct. FEMA doesn't match any of them and ranges from 0.3 above to 0.3 below, and the reference marks by FEMA are all listed with elevations on the FIRM. FEMA says use NAVD88, but the profiles were obviously done using the reference marks.
I surveyed a house slab, and it falls within this discrepancy. Contacted the parish flood plain coordinator, and they do not have a suggestion.
I believe the best thing to do at this point may be to state the discrepancy on the FIRM and let the insurance companies decide, but I am pretty sure this will only cause confusion.
Suggestions?
In my area reference marks are no longer shown on the FIRMs and the instructions on the FIRM say to use 88.
Are you by some chance getting the reference marks off of old maps?
IMHO you should do as per the instructions on the most recent FIRM.
The HEC-RAS profiles of the streams are based upon the reference marks. The flood levels are based upon the reference marks, and they match the reference marks. The maps says datum is NAVD88, but it is in correct. No question about any of that. It is a bona fide discrepancy.
IMO the important part is:
"The HEC-RAS profiles of the streams are based upon the reference marks. The flood levels are based upon the reference marks, and they match the reference marks."
i would base the EC on the reference mark, show it as the primary BM on the EC, and note the difference between it and NAVD88 in the comments area.
you're lucky that the reference marks checked that close between themselves. i've seen everything from typos to flat out pencil whipping field notes to make a bench loop close.
In my area the most recent FIRM's were adjusted to 88 and the reference marks eliminated because they knew they were all over the place. Also, I don't think the profiles were based on the reference marks, but rather the reference marks were just accumulated that were within a reasonable distance of the stream and they were created by many different sources (NGS, USGS, Highway Dept, SCS, etc) wiith many of them having unknown quality.
As I say, this is my experience in my area. Your area may differ. But the bottom line is that the most recent maps and instructions say use 88.
If you feel further explanation is needed, that's what the comments section is for.
If the BFE is based on data collected utilizing those RMs I would consider using them for primary control and checking your datum as other/source on the EC. This would allow you to utilize the FIRMs datum without stating that your data is 88, which you have indicated it is not.
Have you called FEMA to see what they think?
The maps says datum is NAVD88, but it is in correct
It is a difficult question to answer here, just what is NAVD88, the flood maps are based on a lidar topo flown and ground controlled with panels. The elevations for the ground control was based on NAVD88 first order bench marks. Then cross sections were run by someone, it sounds like some sort of engineering students. How they ran them, I haven't figured out yet. If it was OPUS based then all the elevations for the sections are higher than the elevations for the topo and how high depends on what Geoid they used. Anyway, consider yourself lucky to only see .3'. I think what you are seeing is exactly what's happening in my area and I think you are right on. I doubt most surveyors even do any checks at all to figure out where these elevations are taken from.
From what I've seen the old FIRM maps generated much better elevation control and cross sections. After all it was all leveled and the bench marks are put on the maps for reference. Basically a gold standard type of elevation survey, the new ones don't seem to have that going for them. So just how do you get NAVD88 for the FEMA maps, bench marks or OPUS/CORS?
> I have a case where a FEMA reference mark referenced by FEMA varies from NAVD88 by about 0.3 feet. I checked it against other FEMA reference marks and found them to vary between each other by about 0.2 feet, some higher, and some lower. I checked the NAVD88 USGS monuments, and they are all correct. FEMA doesn't match any of them and ranges from 0.3 above to 0.3 below, and the reference marks by FEMA are all listed with elevations on the FIRM. FEMA says use NAVD88, but the profiles were obviously done using the reference marks.
>
> I surveyed a house slab, and it falls within this discrepancy. Contacted the parish flood plain coordinator, and they do not have a suggestion.
>
> I believe the best thing to do at this point may be to state the discrepancy on the FIRM and let the insurance companies decide, but I am pretty sure this will only cause confusion.
>
> Suggestions?
Is this possibly a case where the RM elevations were correct when they were established but have since sunk or risen? I know some parts of LA have some serious issues with that.
Larry P
I would give a call out to Cliff Mugnier...
and discuss this issue with him.
The height modernization discussions I have attended here in central Texas have all pointed to the subsidence issues with Louisiana and "busted"/changing vertical data/monuments.
Of course, the subsidence issue affects Houston, TX but that is not your discourse here.
