This is a rehash of my previous post, as I'm really in need of your input on this one.
Dealing with a couple houses that only measured .2-.3' above the BFE.
I established my NAVD88 vertical with 2 hour GPS observations and used OPUS Static to adjust these and generate reports.
To double check (on the above two homes), I ran two independent Static Observation on two points, 800' apart, with different sky views, that were occupied a week apart, and they matched
within .02' when compared to my turned set traverse measurements with a 3" S6
I have also found (on a good day in the wide open, my Single-Base-Line CORS RTK elevations hit within .1' of the OPUS solutions, though I'm about 20 miles to the nearest station, so more often than not, the CORS RTK is pretty worthless as the vertical creeps around by many tenths when it is being challenged by bad satellite constellations or obstructions.
The closest NGS vertical control that is suitable for a GPS occupation is in a city 8 miles away from my site. There is no vertical control any closer to the property that 7 miles.
The historical and new FIRM lists no REF MARKS.
Q.) Would you suggest I do a simultaneous occupation adjustment holding the city Mark as fixed, to check my OPUS, or in this case would you run with the two OPUS occupations?
I think in the real world a lot of surveyors are hanging their hats on solutions from OPUS, however I am cautious as I do not know what FEMA would demand, if or when I receive an audit on an eLOMA, or if a single report would be good enough.
Thanks
The instructions in the elevation certificate state that you can use an OPUS solution provided you attach the printout from NGS showing which stations were used to the certificate.
The closest NGS vertical control that is suitable for a GPS occupation is in a city 8 miles away from my site. There is no vertical control any closer to the property that 7 miles.
7 miles is a long way to vert control, even out here in the big country. I would tie to it, and I would tie between my control and it, not worry about OPUS. And I would tie some others even if I couldn't set GPS on them.
So far, the jury is divided.
7 miles of leveling with an automatic optical level, or 8 miles (GPS hopscotch traverse line).
Not sure what would be more accurate.
My concerns are mostly due to my pushing eLOMA's off the OPUS benches and to date I have not one FENA response for my over 30 day old submissions.
I am care-free if I have a couple feet of free-board, but the razor's edge crowd has me procrastinating to no end.
This is why I have posed the question to the Oracle that is surveyorconnect.com
I wouldn't run long levels, I've got the same issue, can't occupy official bench marks. Some are set vertically in walls, I just set a GPS point near, then level from there. I would tie between them using static and not include OPUS or CORS. I would see how that checks, but you want to know how the bench system checks to it self using the geoid model. It really shouldn't take much time.
The FEMA maps in my area only show the BFE to the nearest foot. Are you interpolating the tenths or are your maps publishing elevations to a tenth? The BFE's are computer generated from a model using contours that are not accurate to a tenth of a foot, probably not even to a foot. Why would .2 be a concern? I've never found a FEMA official that was informed enough to explain that to me. Maybe someone here can.
You should be determining the Base Flood Elevation (BFE) from the Flood Study, not the maps.
I am using the FIS reports to find my BFE data.
My county only has a few lakes that are addressed by the 2014 FIS.
I am also using any published ELOMA's resulting in removal from the FSHA, that have included BFE/1% Flood Elevations if I am in a Zone A without a study.
These are all to the nearest 1/10th of a foot.
I suppose I am going to need to do my own differential GPS based on the hard marks.
What type of results have people found when tying into multiple bench marks across a network?
Has anyone ever leap-frogged a Base and Rover RTK pair in order to level into a site from known control?
Since I have 2 heads, but no adjustment software, my options would be OPUS-Projects or RTK leap-frog.
You can download the GNSS Solutions post processing software, or Topcon Tools and use the Demo mode to process your static data. The GNSS Solutions will process the L1 data for free. The Topcon Tools software will process L1/L2 data, up to 5 points in Demo mode.
I would not trust the RTK for transferring elevations.
I would run several static sessions, and different times of the day to have redundant sessions. You can also use OPUS as a blunder check.
I use the same method that Moe uses when I cannot occupy a benchmark either because of canopy, or because of location, etc.
OPUS Projects!
Add more CORS to your network of simultanious static sessions. Study the CORS elevations along with your observations of the NGS benchmarks. Constrain to the 'best' and publish your mark in OPUS DB. Let FEMA prove you wrong!
DDSM
Sorry Broken, my response was to Skeeter's comment questioning the tenth of a foot Base Flood Elevation. I assumed you were using the Flood Study.
What I have done in my area is to tie OPUS observations into the few remaining FEMA reference marks. Fortunately we have a lot of CORS sites in my area, and a good geoid model. Unfortunately all the maps and studies in my area are still based on NGVD'29.
Even with having to use Corpscon to convert, I have found OPUS to be within a tenth and most of the time less than a tenth of the FEMA reference marks.
My first choice is always a level run from a FEMA Reference Mark, but when that's not possible I know from checking that OPUS will work in my area.
I would run OPUS observations on any Reference Marks you can find, if you can't set directly on them set a point near by and run a level loop to them. Then check your OPUS results to the points published elevation.
Good luck.