Current FIRM for nearby town no longer shows the RMs from the older FIRM. ?ÿHowever, the elevation increase shown for cross-sections is identical to the old values plus the delta between 29 & 88.
Yesterday I went out and did a longer DPOS (Javad's OPUS) observation and checked into the other surveyor's benchmark and an old RM from the previous FIRM.?ÿ I'm hoping the old benchmark will convert and prove the new elevations correct (within a reasonable amount).
I haven't spent much time researching other town FIRMS. This morning I went onto the FEMA website to see how to do it, it's much easier than it used to be.
I looked pretty quickly at six Montana towns, 5 of them reference bench marks and show them, however the panel for Bozeman only shows 3 bench marks which is problematic. I don't see any for Whitefish, maybe I'm missing them but I didn't see any.
I was able to repeat my previous results within 0.12'.?ÿ I belive this is the difference between OPUS and DPOS.?ÿ The check shot on the "benchmark" was 0.29' higher than the NGS elevation (converted from meters).?ÿ My main issue is the NGS benchmark has the elevation listed as "Aprox. Height" with the elevations shown to the tenth of a meter and to the foot.?ÿ I would assume that these elevations are not accurate enough to be relied upon for an Elevation Certificate.
Just happy I was able to repeat my previous work within a reasonable tollerance from a different location.
This is a benck mark or a trianglation station?
I am not sure that it's a benchmark and I am not sure 0.12' is a reasonable tolerance. NGS bench marks will give you a network accuracy estimate.
When searching for bench marks on the NGS web site one of the options for verticals is approximate heights: an open circle, the only time I've seen the open circle is for horizontal control points. The triangle for a horizontal site and an open circle for low quality elevation control and a triangle with a dark circle for horizontal control points that have been leveled. I wouldn't put much value on an open circle point for vertical control, often they are old NAD27 tri stations that were trig leveled from many miles away and then converted from that poor quality NGVD29 value to NAVD88. Here is a classic Horizontal point that was leveled in a first order run, then turned into a HARN point, the tri station and the reference monuments have good elevations.
and a classic horizontal point with approximate heights. Not a point to use for FEMA
trig leveled from many miles away in a messy GEOID area.
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Perhaps some are unaware that the CORS stations used in OPUS solutions are, in and of themselves, NGS reference points.?ÿ I am sure that policy has evolved but ten years or so ago when TN-DOT was bringing their state-wide network online, it is my recollection that they submitted 30 days of continuous 1-second epoch interval data to the NGS, which was post processed to verify the positional accuracy and stability of the reference station antennae in order to have them accepted as CORS sites.
As a result of meeting these thresholds, CORS sites are in my opinion better reference points than terrestrial monuments because not only does NGS know where they were when accepted, but has continuously monitored their position since then.
The reference datum is in my opinion a place to look to sort out the difference in elevations.?ÿ If that is not the issue, I wonder how the other surveyor obtained his positions.
I'm using the term "benchmark" in a broad way.?ÿ On the NGS website this was an open circle with the filled in triangle.?ÿ NGS calls it "GPS and Approx. Height".?ÿ Again, I would not accept it for an accurate elevation, but the other company did.
The 0.12' differential I believe is from the OPUS vs DPOS.?ÿ I chose to accept the OPUS report this time to make sure the other company knew what I was referring to.?ÿ My first DPOS was at the site of the Elevation Certificate.?ÿ This second one was from a mile west, so I could check into the "benchmark" the other company was using.?ÿ Since this was an RTK shot 1 mile from my base, I thought the results were close enough to verify that I did not have a 1.5' bust in my previous work.
I have since reported that I could not find an error on my part, and that he should double check his info.
No open circle point on the NGS website should be used as elevation control for a BFE.
Are there any County or City Benchmarks you can use? You need to find a know point you can use that has an Elevation then go back and check the TBM you used for the Elevation Certificate.?ÿ It seems you have tried to do this, and I am sure you feel better about having a 1.5 foot mistake, but you still need to find a benchmark you can check into.?ÿ
Are there any County or City Benchmarks you can use? You need to find a know point you can use that has an Elevation then go back and check the TBM you used for the Elevation Certificate.?ÿ It seems you have tried to do this, and I am sure you feel better about having a 1.5 foot mistake, but you still need to find a benchmark you can check into.?ÿ
I agree with Scott and would check to see if the City and County have benchmarks, especially if either has a flood plain management program that would ultimately be reviewing the elevation cert.
I agree with Scott also, benchmarks even 6 miles away will be a good sanity check on your GPS#'s.
Small city with no benchmarks.?ÿ County does not run any kind of survey control.?ÿ DPOS (OPUS) was the only reasonable option.
I looked up the 2015 FEMA flood cert and it says Datum used for building elevations must be the same as that used for the BFE.
It also asks for the "benchmark used"?ÿ This would seem to mean the benchmark used would be the same benchmark used for the BFE elevation on the FIRM. Good luck with that from my experience.
If nothing else this thread has demonstrated that two elevation certificates done for the same purpose could easily have two answers. I certainly would not tie to an approx. height control point but after that how do you determine the BFE datum from historic flood maps without a lot of work to correlate the map to surveyed features on the map.?ÿ It doesn't seem to me like you can seriously?ÿcertify the nearest NGS vertical control benchmark or an OPUS height is on BFE datum without supporting evidence.?ÿ High risk - Low reward.
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Agreed!
There is another resource: If FIRM panels are older than 2001, review the FIS report; if newer maps review the Technical Support Data Notebook (TSDN) for SURVEY and TERRAIN.
The Survey TSDN contains metadata on the methods and equipment used to establish the control used for the structure and cross-section field surveys (includes which NGS benchmarks were used and temporary points established). This notebook also contains a spreadsheet of EVERY point measured.
The Terrain TSDN has additional benchmark data used during the LiDAR check shot surveys.
Here is a link to example TSDNs:?ÿ http://coloradohazardmapping.com/Data
And remember that the FEMA EC supports OPUS: https://surveyorconnect.com/community/surveying-geomatics/two-differing-elevation-certificates/paged/2/#post-469628
DDSM
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The FIRM panel will have a datum called out. If you use OPUS you are supposed to say so and include the report as an attachment.
There are two considerations. One is repeatability and the other is a relationship to the regulatory information on the map. One very important factor to remember is the map is for regulatory and insurance rate use. The numbers may have little to no relationship with actual risk. If you derive a correct number for the datum and provide the report you are good.
