The newly revised FEMA flood maps have recently been issued for my area, and the number of phone calls requesting elevation certificates has increased. I frequently pass on these unless they are in an area where I already have benchmarks from previous work. Reliable benchmarks are getting hard to find. I just spent an hour searching NGS, Mass Highway, etc. for a call this morning, and the nearest benchmark I could find is at least 3 miles away (I know you guys in Texas and Alaska are probably thinking "Only 3 miles?" but in suburban Massachusetts, 3 miles is A REALLY LONG WAY!)
I've always thought I should only use a published elevation (like NGS, USC&GS etc) for this work (if it's not a bronze disk, I don't trust it!). The original 1988 flood maps had benchmarks (RMs) on them that I always tried to use when those flood elevations were in effect, and there is one from 1988 right around the corner from this particular site, but it is on a hydrant spindle (the top nut that operates the hydrant). In fact, every RM within reach is on a hydrant spindle. These I have always found unreliable even in the 1990s (by several tenths usually), and in the last 26 years since the map was published, most of those hydrants have probably been replaced.
This particular town has an engineering department that has their own benchmarks throughout the town, usually on culvert headwalls. But these were mostly run in the 1970s it appears, and who knows how well they were run.
I don't have GPS (can't see the sky very much here - too many trees!), but I was considering it for the future mostly for this purpose (elevations). Is GPS reliable enough for elevations to use for elevation certificates?
Keep in mind that many of the houses that I have done certificates for have been just 1 or 2 tenths above or below the base flood elevation, so the reliability of the benchmark is critical.
So my question is: What level of reliability do YOU use for these benchmarks?
It depends on the City I am doing the elevation certificate in. Most of the time the City wants it tried to their benchmark. I also use County Benchmarks or NGS benchmarks
Short answer is that FEMA requires all in NAVD 88 datum. In the note section you just state you used VERTCON or OPUS to generate to 88. Never a problem in this zip code, and they are pretty helpful.
It's all a joke anyway, so I'm totally with you on not doing any more. Other clowns in town seem to think they are worth half of what I would charge.... bye bye
I use the GPS to establish an NAVD elevation at or near the site.
Yesterday I recovered a MassDOT pin in a swamp, leveled out to a stake in a median and set a receiver there. It's within a mile of locus.
I then set two points on the subject property and leveled through them to a turning point. I setup receivers on the two points.
I will download a static file from MTS which is within 4 miles.
I may also download the MassDOT CORS data (17 miles and 20+ miles). If I do, I will also download the MTS data that is about 20 miles away.
My 3 occupations were for the time it took me to level around the house, measure it, check my level loop closure, measure from the GPS units to several building corners, make a phone call to the realtor to try to get in and take pictures of the house.
Set a benchmark on site. Establish an elevation from:
1) Cell RTK connected to CORS, two separate observations averaged within a set tolerance.
or
2) Static GPS. Gather data for a couple hours while doing other work, upload to OPUS, receive elevation.
The new OPUS X90 unit is paid for with 2 jobs. Worth it!
The procedure you use will only yield ellipsoid height to the reliability of your methods and instrumentation. Once that is achieved, you STILL have to then compute elevation based on the current geoid. The reliability of that varies from place to place, so you better have sufficient experience to know whether you can bet your license on that. It's OK if you have experience in your geographic area of practice, but it's NOT for the neophyte with a brand-new GPS rig and a whole weekend of lectures at the Holiday Inn Express.
I wouldn't publish an elevation certificate (here) based on a short static processed through OPUS. It is erroneous to assume the quality indicators on the sheet translate to 'maximum' or even 'probable' error. I routinely get two solutions on different days where the elevation difference is 3 times the RMS given on the sheet. I have seen up to a decimeter with two hours of data. That being said, in most urban areas there is a TON of static data available for download. Boise is no exception. I can get public files and be sub-centimeter with an hour of data.
The trick to using GPS for elevation is finding what works in your area. Some places your only option will be extended static. Others you can get 4 VRS based solutions and be tight a few centimeters or less. In some areas the geoid model is so crappy you're better off leaving it at home.
I suggest picking the brain of some locals followed by experiments with a rental system. We can give you good basics here but your mileage will vary...
Then on the form
They tell you, in the instructions, to round to the nearest tenth.
You also have to attach your OPUS results to the form.
OPUS might be all you can reasonably do, if you can't find any other way. The EC form does allow you to report using an OPUS observation, but I agree with Cliff its risky if the elevations are going to be close.
I would certainly try a network rover that I could run a small network around the property on first and then check with a spirit level. But you have to have access to that equipment first.
Did you get the Flood Study from the FEMA web site? Was the stream actually studied (are there base flood elevations?)or was it modeled off the topo maps or DTM's?
If yes (it was studied), get the study and find out who actually did the study for FEMA and see if they have any information to share. Here in Illinois, IDNR Water Resources has usually done the studies for FEMA, but that will vary state to state. Cross check the structures shown on the study nearby to see if they agree with your elevations. The ultimate goal here is to get on the same datum the study was done on - that's what's really important and a bigger concern than whether it's perfectly conforming to any one party's idea of what NAVD88 is for this area. Everything should have been done on NAVD88, but you will find in your lifetime that some versions of this are different from others. Try to find one that best fits the structures or other hard surfaces shown on the flood study.
And all for $750! 🙂
Cliff,
"The reliability of [the current geoid] varies from place to place..."
How DO you determine whether or not the OPUS Ortho Height is "good enough"? What sort of checks would you use before betting your license?
Oh, and I missed the weekend of lectures at the Holiday Inn. Do they have those often? 
Dave
NGS suggests performing a perimeter survey of benchmarks (for instance - a county) to see if the slope of the geoid appears to match the slope of the actual orthometric heights published by NGS. They have an entire publication on GPS determination of Orthometric Heights; I just briefly alluded to the verification process recommended by NGS.
It's for situations exactly like this that I had a benchmark installed on my truck bumper.
When did you all start speaking in Greek???
Thanks for the replies. I understood Scott Ellis! The rest of them...well, I can only hope that someday I will reach the status of GPS neophyte, but I doubt that I will will have time for a whole weekend of Holiday Inn lectures...:'(
Don't worry, I had already declined the job. In fact, I think I will continue passing on these. The homeowners are in a bind, but they still don't want to pay more than a few hundred for the certificate. It's hard explaining to them how much time it will take to get a benchmark run to their site. It's just not something I think I can make a profit on, not when there is real work to be done, anyway!
Maybe if I was a neophyte...
Now I have to go look up OPUS, MTS, CORS, RTK, X90, ellipsoid, geoid, ortho height, static, VRS....
Where did you guys learn all this stuff? I don't know of anyone local that has an understanding of the subject that a lot of you do. Most of the people I know around here that have it just push the buttons and read the numbers.
I would pay quite a bit for a working truck bumper benchmark!:-P
Cliff,
That looks like some heavy reading. I better put on a pot of coffee:
https://www.ngs.noaa.gov/PUBS_LIB/TRNOS110NGS32.PDF
Dave
Dave,
No different from the stuff I publish; my students say my pubs are sure-fire cures for insomnia.
4th Order (or better) 'Survey Control Data Base' marks. Commonly know here as PSMs (Permanent Survey Marks.) Information on which is maintained by the state and are all (supposed to be) on A.H.D (Australian Height Datum)
I use my GPS RTK network to set control, I observe the control 3 times under different satellite geometry. My Geiod is based on the 09 version. Most times I tie into town control (city benchmark) as a check.
I've checked my Geiod around my area against NGS, CT DOT, and USGS Mons and I've hit them all within a tenth or so vertically, so I have confidence in the geoid. Our state has set out MTS on GPS, so I follow those plus, if not go beyond them.
Most people are so below the BFE around the coast in SW CT that a tenth wont make a difference.
Connecticut Assocation of Land Surveyors seminar
If you have a chance attend a CALS seminar on GPS. Professor Mayer(sp) is great at explaining GPS and geiods.
