Ok another dumb question..for contractors or engineers

[Autotech]

I had and elevation done on my lot last year ;cost me 400.00 bucks because its in a flood zone. Lot is 145.6 ft above sea level according to Bench Marks Certified. got to be 160. Now here is the question. My GPS elevation shows 176 at Bench Mark. I know a GPS is not perfect but I can't belive its 30 ft. off. I've been to places before that gave the elevation and looked at GPS and it was within 5'. Is there a different scale that is used or just better equipment used by engineers/surveyors.

 

[Barnacle_Bill]

Re: Ok another dumb question..for contractors or engineers

Elevations are not very accurate. http://gpsinformation.net/main/altitude.htm

 

[mommicked]

Re: Ok another dumb question..for contractors or engineers

Find another benchmark and chek it to your GPS,or two to really prove it. Benchmarks are public knowledge on FEMA maps.surveying GPS devices can get very precise when set for a longer period of time.constantly tracking multiple satelights and averageing.local half mile or less homeowner foundation elevations are frequently checked w an old fashioned level and level rod,traversing there and back,for a check on, and proof of the precision of the difference in elevation.Posted Fema elevations are rarely wrong and never by 30' unless its Mt Saint Helens or something like that!!However ...... upstream development can change historic flood conditions downstream.see hard surface and runoff.and fema maps are not updated regularly.its very expensive.places that have rarely flooded can begin to, w to much runoff from upsteam developement.its engineered not to ,but its not an exact science.could be a mistake by the Company you hired or personell in translation.but i would think they would chek again If you question them w your findings or show proof of their precision.I have seen some foundations that are in the flood elevation before.it doesnt happen alot but older houses are more prone to it.for various reasons.usually the homebuilder. but they were usually built years ago and no recourse now.Liability.ocassionally fema raises the flood elev.in areas making old conforming foundations now under the flood elev.then theres the whole 100yr,500 yr flood thing i think is silly as there has obviosly been developement and or man made obsticles to flood waters almost everywhere since 100 yrs ago.and the maps have dates on them,lots are old, and alots gone on since then.forcasting the hight of floods is a very complex issue.only mother nature knows how high floods may rise, we can only get so close w our estimates!!A hurricane or 2weeks of rain can make most any lowlying area flood.thats why land is usually cheaper in these areas.we had issues here after hurricane Fran because all of the trees that fell in the floodplains blocked flows and dammed creeks and caused more flooding by holding back flows for yrs after.some areas I hunted became swamps that were once dry.

 

[Bondo]

Re: Ok another dumb question..for contractors or engineers

Ayuh,.... I have a pretty decent GPS in my boat,....
It rarely to Never get's the Elevation of Lake Ontario, Correct....

Now if ya got a Surveyor's, or construction Jobsite GPS,... That's different,...
Much, Much more accurate...

I'm liftin' 1 of my houses about 12' to get outa the flood plain...

 

[KRS]

Re: Ok another dumb question..for contractors or engineers

And, honestly, it doesn't matter. Their certification is the final word when you're in a flood zone, particularly if you're going for an elevation certification to get out of the flood zone.

You could (and you're not saying you're trying to) argue til you turn blue, but it won't make a bit of difference due to the gov't beauracracy.

 

[marlboro180]

Re: Ok another dumb question..for contractors or engineers

KRS- gotta agree with you on that one. I am blue in the face, but I am still trying to get some sense through to them.... Probably to no avail, but what the heck....

Been there done that, or should I say still doing that. H&H modeling changed the FIRM map here on my property , and now my place is in the floodway! Dreaded zone X!!! I raised my house well over 2' over RFE, and they still are arguing about it, as in it is not high enough.... And I am using a monument 300' from my house!!!!!

Autotech- find you nearest monument, and work off of that. You have a full topo survey?

 

[Autotech]

Re: Ok another dumb question..for contractors or engineers

Thanks Guys I'm just jumping through more hoops for the buracrats. Now they say I need another certification on the foundation along with county admin. office letter of inspection. Just another 400.00 bucks out of pocket for no reason in my honest opinion. If they would come look at the BM and the floor its a no brainer I'm 4 foot above flood plane. Oh well how high..

 

[marlboro180]

Re: Ok another dumb question..for contractors or engineers

Ok, so BM is what, base map? Basement ? Or Bowel Movement?'Cause the latter is what it is all about sometimes, GoogleEarth will get you in the ballpark, or if your county is in the GIS system that works too. Only for a ballpark elevation though.

They want a certification letter of inspection because they are covering their liability ( FEMA) that they approved the plans and construction in a floodplain, and they want to make sure it was done properly in accordance with floodproofing a home, new or old construction. You are building on a levee, so expect a few dollars being spent on things that seem extraordinary.

I have spent tens of thousands on projects to get them done according to their rules, but they got done. 400 bucks, on the grand scheme of things, is not too bad at all, make 'em happy, and move on. 'Ell, the Onan cost more than that, hey??

I am not dismissing you,nor your concerns, lots of things do not make sense, but pay 'em , get the certrification, inspection, and lets see some pics of the treehouse . Still waiting for the diving board photos!

edit- added....

Hey, for what it is worth, I have done several residential rebuild projects in zone AE, and Zone X. Not Once did the DNR ever come to any site, EVER, to check to see if what was done conformed with the usage and elevation requirements, even though the permitting process on each one was onerous, to say the least.

 

[Autotech]

Re: Ok another dumb question..for contractors or engineers

Ok, so BM is what, base map? Basement ? Or Bowel Movement?'Cause the latter is what it is all about sometimes, GoogleEarth will get you in the ballpark, or if your county is in the GIS system that works too. Only for a ballpark elevation though

Bench Mark last survey guy put on the lot.
Yep learned last they say we have to have this, before we can do this, and I do this, then I have to have that ,then i do that ,and I now they want this too, and so goes the cycle of shuffeling paper so someone can justify their job in the govermental bureaucratic system. Makes life fun. Therefore I just try to be nice and persistant. Get nasty an they will sit on it that much longer. Ya'll have nice day....

 

[mommicked]

Re: Ok another dumb question..for contractors or engineers

Sorry If I wasnt clear on that.FEMA makes all the Flood zone maps in the country.These maps all have a Fema bm.Bench mark on them.usually on a permant concrete headwall at a bridge or creek culvert under a road.these are used by surveyors to determine elevations on lots of surveys including residential flood certification surveys.Ive always done them w a level rod and level.we measure from the BM to the found. and back by adding and subtracting the elev read on the rod,backsite,frontsite,move ahead of frontsite,repeat,there and back to the starting point.w no errors you should have the same elev.when you return to the starting point.thats the check of the precision of the level loop.the elevation of the new BM at your prop.is then computed from the math and posted.Im not sure if GPS elev. are allowed for these certifications,and Im NOT a registered surveyor,But I have worked under several.And I slept at a Holiday Inn last night!

 

[JB]

Re: Ok another dumb question..for contractors or engineers

Generally, recreational GPS is accurate to +/-30' (or better). Seems likely to me that includes the vertical component of the fix as well as the X and Y.

 

[dockwrecker]

Re: Ok another dumb question..for contractors or engineers

If this was a licensed Surveyor he's probably done the following: If he brought in your elevation from a monument only 300' away, he also checked into another monument somewhere as standard procedure. Benchmark elevations can be several things beyond survey monuments. Manhole covers, top back of curb at intersections, storm drain grates, etc that can be used to verify that the monument hasn't been altered. So once he's done that, most of the time he'll shoot it in with a simple rod and eye level accurately to 500' with a =/- of .01'. The cool guys now use the GPS finders (that also cost about 15K) that still must be checked to a benchmark but are accurate to =/- .005 over several thousand feet. Either way, any surveyor (or competent field engineer) can easily establish a benchmark elevation on you property within 1/8" from a known benchmark at 300'. I've done it regularly for 15 years. It's not hard at all with the proper gear. But what he has that you don't is the STAMP that says he's right. I work closely with surveyors on a daily basis as a General Superintendent and never start a job without certified control points.

 

[marlboro180]

Re: Ok another dumb question..for contractors or engineers

Sorry If I wasnt clear on that.FEMA makes all the Flood zone maps in the country.These maps all have a Fema bm.Bench mark on them.usually on a permant concrete headwall at a bridge or creek culvert under a road. !

Not to quibble ,but FEMA does not make the maps, they use them. FIRM maps determine the zones in a floodplain, determined by the height of the RFE in , say, a 100 year event.But, then again, FEMA hires the contractors to determine the base elevations, and that is now done by flyover . +- 1 to two feet differential in elevation. Then it all gets plugged into a computer, with somebody trying to make sense of it all, then the results get spit out. H&H......at its finest.

Then the local municipality ( in conjunction with the county) tries to determine what it means, how to enforce building codes there, and whether or not it can be built or improved upon, and to what extent improvements can be made.

I am leaving out a lot of details, but this is the gist of it. And Autotech, I would go find your nearest monument and check its elevation and make sure it matches what they have in their system......The surveyor should have used a monument, and the benchmark loop ,as Dockwrecker described, especially in your area..

' night too.