Elevation Certificate - Benchmark ties

Seems like you answered your own question. Some might think you are being too anal, some may think you are not being anal enough! If you aren't comfortable, don't stamp it. Period.

I know of some surveyors who think a plus or minus a foot indication on a road intersection on a USGS topo is good to go for control. For myself, redundancy is required. I typically identifiy all bench marks within a mile or two of the project before field work. I want three tied, by whatever means you have available, to make an initial determination of certifiable elevation. You want cheap, hire whichever whore you wish. It's not me.

I don't think so. FEMA used to put Reference Marks (RM's), on the old FIRMS that you were expected to use. They don't do that anymore but instead require you to reference 88 elevations. They show no benchmarks on the new FIRM's.

Why did they take Reference Marks off the maps? Because they knew that in number of instances they were wrong.

So if they didn't like them, why should you?

Item C2. A field survey is required for Items C2.a–h. Most control networks will assign a unique identifier for each benchmark.
For example, the National Geodetic Survey uses the Permanent Identifier (PID). For the benchmark utilized, provide the PID or other unique identifier assigned by the maintainer of the benchmark. For GPS survey, indicate the benchmark used for the base station, the Continuously Operating Reference Stations (CORS) sites used for an On-line Positioning User Service (OPUS) solution (also attach the OPUS report), or the name of the Real Time Network used.

[sarcasm]Just use OPUS...that is how the Map Modernization 'data collection' (field survey) was controlled.[/sarcasm]

DDSM:-S

> Item C2. A field survey is required for Items C2.a–h. Most control networks will assign a unique identifier for each benchmark.
> For example, the National Geodetic Survey uses the Permanent Identifier (PID). For the benchmark utilized, provide the PID or other unique identifier assigned by the maintainer of the benchmark. For GPS survey, indicate the benchmark used for the base station, the Continuously Operating Reference Stations (CORS) sites used for an On-line Positioning User Service (OPUS) solution (also attach the OPUS report), or the name of the Real Time Network used.
>
> [sarcasm]Just use OPUS...that is how the Map Modernization 'data collection' (field survey) was controlled.[/sarcasm]
>
> DDSM:-S

I suppose if I were in New Orleans, I would trust the OPUS solution over the local benchmarks, but in my area OPUS can be 0.3'+/- last time I checked.

The Bow Tie Surveyor

> I suppose if I were in New Orleans, I would trust the OPUS solution over the local benchmarks, but in my area OPUS can be 0.3'+/- last time I checked.
>
> The Bow Tie Surveyor

The text I posted is directly from the Jan.13 Elevation Certificate INSTRUCTIONS.

I suggest requesting the Technical Support Data Notebook (TSDN) for the 'data capture', 'recon', 'field surveys', and 'terrain'. This document will tell you which 'benchmarks' and methods were used.

DDSM;-)

> > I suppose if I were in New Orleans, I would trust the OPUS solution over the local benchmarks, but in my area OPUS can be 0.3'+/- last time I checked.
> >
> > The Bow Tie Surveyor
>
> The text I posted is directly from the Jan.13 Elevation Certificate INSTRUCTIONS.
>
> I suggest requesting the Technical Support Data Notebook (TSDN) for the 'data capture', 'recon', 'field surveys', and 'terrain'. This document will tell you which 'benchmarks' and methods were used.
>
> DDSM;-)

Yeah... Maybe they should list the 95% confidence interval next those BFEs that they show on those maps. I worked on an EC the other day where a lady's FF was 2' below BFE on the maps that were revised in 2008. The 1983 map showed her out all together. Called the counties stormwater engineering department about it and they said that they were proposing a 3' drop in the BFE in the area on the next round of maps due to soil conditions. Three feet is a lot of adjustment in my opinion. It tells be that the BFEs on those maps are just rough guestimates that they are using to regulate by.

The Bow Tie Surveyor

> > > I suppose if I were in New Orleans, I would trust the OPUS solution over the local benchmarks, but in my area OPUS can be 0.3'+/- last time I checked.
> > >
> > > The Bow Tie Surveyor
> >
> > The text I posted is directly from the Jan.13 Elevation Certificate INSTRUCTIONS.
> >
> > I suggest requesting the Technical Support Data Notebook (TSDN) for the 'data capture', 'recon', 'field surveys', and 'terrain'. This document will tell you which 'benchmarks' and methods were used.
> >
> > DDSM;-)
>
> Yeah... Maybe they should list the 95% confidence interval next those BFEs that they show on those maps. I worked on an EC the other day where a lady's FF was 2' below BFE on the maps that were revised in 2008. The 1983 map showed her out all together. Called the counties stormwater engineering department about it and they said that they were proposing a 3' drop in the BFE in the area on the next round of maps due to soil conditions. Three feet is a lot of adjustment in my opinion. It tells be that the BFEs on those maps are just rough guestimates that they are using to regulate by.
>
> The Bow Tie Surveyor

That situation, where the property was shown as "out" on the 1983 map (I assume the house built prior to 2008, but after 1983), should qualify for "grandfathering" ... that is, at least until FEMA "phases out" the grandfathering rule starting later this year.... But I think that if she get's a grandfathering rate in place now, and it is her primary residence, then she *might* be able to keep that rate indefintely.

You've answered your own question.

I found myself in a similar situation not too long ago. Had I known about this forum I would have posted about it. I was asked to provide elevations on a number of wireless towers for completing FAA Form 7460-1. The form required the elevations to be accurate to within 1 foot. The kicker was that the FAA was requesting that the vertical datum be NGVD29, same as the 60 + year old USGS topo maps. Problem was that hardly any reliable benchmarks have survived from that era, and the NGS is no longer publishing the 29 elevations on any of their vertical control. No NGS conversion tools. I'm having visions of an airplane colliding with a cell tower by my measurements being off.

I ended up doing a systematic search and eventually recover 6 surviving benchmarks on over a hundred miles of the Parks Highway and completing static sessions on them to derive a standard deviation between the old 29 and NAVD88 datums. Five of the six benchmarks agreed within +/- 0.3' of each other and the sixth was an outlier and I pitched it. I ended up with a good sense of the seperation between the two datums based on my own measurements.

Do what you gotta do.

The NGS data sheets still list NGVD29 elevations under Superseded Control for those marks that had them. These should still be the same numbers that were on them before NAVD88 elevations were established.

> > > > I suppose if I were in New Orleans, I would trust the OPUS solution over the local benchmarks, but in my area OPUS can be 0.3'+/- last time I checked.
> > > >
> > > > The Bow Tie Surveyor
> > >
> > > The text I posted is directly from the Jan.13 Elevation Certificate INSTRUCTIONS.
> > >
> > > I suggest requesting the Technical Support Data Notebook (TSDN) for the 'data capture', 'recon', 'field surveys', and 'terrain'. This document will tell you which 'benchmarks' and methods were used.
> > >
> > > DDSM;-)
> >
> > Yeah... Maybe they should list the 95% confidence interval next those BFEs that they show on those maps. I worked on an EC the other day where a lady's FF was 2' below BFE on the maps that were revised in 2008. The 1983 map showed her out all together. Called the counties stormwater engineering department about it and they said that they were proposing a 3' drop in the BFE in the area on the next round of maps due to soil conditions. Three feet is a lot of adjustment in my opinion. It tells be that the BFEs on those maps are just rough guestimates that they are using to regulate by.
> >
> > The Bow Tie Surveyor
>
> That situation, where the property was shown as "out" on the 1983 map (I assume the house built prior to 2008, but after 1983), should qualify for "grandfathering" ... that is, at least until FEMA "phases out" the grandfathering rule starting later this year.... But I think that if she get's a grandfathering rate in place now, and it is her primary residence, then she *might* be able to keep that rate indefintely.

Yeah, that is what I told her. I got her a letter of built in compliance from the county to get the better rate. Nobody is telling her to get flood insurance right now (yet). She was mostly interested because it was affecting the marketability of her house that she was considering selling.

I'm still trying to get my mind around NFIP insurance rates. Like why is a X zone policy cost more than a preferred rate policy? If you are in Zone X, shouldn't you automatically get the Preferred Rate Policy?

BTW, Pseudo Ranger, do I know you? I think I read once that you attended UF possibly around the same time I attended. Just curious. Drop me an e-mail if you like.

The Bow Tie Ranger

I use a RTN system, occupy the points twice and verify it onto local benchmarks.
Works for me.

> The NGS data sheets still list NGVD29 elevations under Superseded Control for those marks that had them. These should still be the same numbers that were on them before NAVD88 elevations were established.

Yes, but, the NGS never published 88 values on any of these old 29 'Superseded' BMs identified on the quads, at least not in the Matanuska Susitna Valley. They more or less just forgot them as they felt they were no longer needed. I found 1 BM with both values published for it, established by DOT/PF and not NGS and used it as a check. I didn't feel comfortable relying on just that one BM, fifty miles away. Having 3-4+' of snow on the ground at the time didn't help much either.

Ain't it grand. Worrying over hundredeths when the data your reconciling against (the flood study) is lucky to be within 2' when the design storm really happens. Yes, we (the public) have to draw a line somewhere to assign a risk of flooding (and who pays), but the reality is that the nats-ass elevation of a structure is the least important element that goes into the determination. Unfortunately for those of us preparing the Cert's, its the only thing that can be verified after the fact.

So let me ask a couple more questions. How was the topographic data that has been incorporated into the flood model generated? Was it based on "technically" incorrect local benches? (here in Vermont, it was) Or was it generated from satellite-based orthography? LIDAR? Knowing the answers to these questions might help one better assess how stressed they need be over reconciling the datum. I guess we're lucky up here: flood maps old and new call out BM's, either by description & elev (old) or PID (new). Can't really go wrong using the one, nearest bench.

> Or was it generated from satellite-based orthography? LIDAR? Knowing the answers to these questions might help one better assess how stressed they need be over reconciling the datum. I guess we're lucky up here: flood maps old and new call out BM's, either by description & elev (old) or PID (new).

I suggest requesting the Technical Support Data Notebook (TSDN) for the 'data capture', 'recon', 'field surveys', and 'terrain'. This document will tell you which 'benchmarks' and methods were used.

DDSM;-)