@skeeter1996?ÿ ?ÿ ?ÿI said nothing of the FIRM elevations being established by using NGS benchmarks, but if you read the attached screen shot of a FIRM panel, one could make that argument.
I believe I said, "reference a published BM and use the same datum from which the FIRM BFE was established".?ÿ The line immediately prior to item C2. a) of FEMA Form 086-0-33(12/19) states, "Datum used for building elevations must be the same as that used for the BFE".
To answer your question, YES, you can use OPUS derived elevations. The instructions for this are:
Just because it is an option, that doesn't guarantee that the approving authority will accept your methods.
I gave my opinion on how to keep your liability to a minimum.?ÿ If you are continuously have 1' difference from your OPUS solutions and published benchmark elevations you might be able to argue that your procedure is more accurate than those used used to establish the published benchmarks. Then again, one could argue that since you continuously have a 1' difference, there may be something wrong with your data processing procedures. Good luck with whatever you decide.
If this site is in Craig, Montana (as you mentioned), then clearly you are not looking at anything FEMA:
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FEMA FIRM Map Number: 30049C1425E (9/19/2012); Community: 300038 (Lewis & Clark County, Montana)
Has this nice little blurb in the lower left corner of the map:
And also clearly shows six (6) NGS benchmarks that have current NAVD88 & legacy NGVD29 orthometric heights (1st Order; 2nd Class) from adjusted leveling in 1991:
SS0852 (W 324): https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=SS0852
SS0849 (V 324): https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=SS0849
SS0848 (W 9): https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=SS0848
SS0845 (Q324) : https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=SS0845
SS0844 (V 9): https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=SS0844
SS0843 (U 324): https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=SS0843
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If that is indeed the case, then you, sir (ma'am), are a charlatan...
@kevin-hines It's not always 1 foot. Sometimes 0.1 foot, once 4.9 foot. I basically know where the data comes from to get a OPUS derived elevation. Not so for a NGS benchmark. Most of the ones I've measured are of the 1960 vintage. I'm relatively sure NGS has not physically resurveyed them to update the elevation from NAVD1927 to NAVD1988. They just ran them through a computer program. Technology and Knowledge about the Geoid has changed significantly since the 1960's. The Geoid is just geodists' best guess of the earth's gravitational and rotation that best fits precise elevation calculation. It's being refined all the time.?ÿ I've not seen any data that shows that NGS benchmarks used any kind of a geoid model.
I've attended many continuing education seminars. Some were given by NGS representatives, some by FEMA representatives. The message I got from the NGS representatives was, "It doesn't get any better than OPUS". NGS is promoting GPS on Benchmarks. That makes me suspicious of the NGS Benchmarks.
I'm leaning towards only using OPUS for FEMA Elevations. I've been somewhat surprised of the support NGS Benchmarks are getting. Most every Surveyor in my area is verifying the NGS Benchmarks with an OPUS observation.
@kevin-hines Usually construction plans have benchmark designated as the control to be used for elevations. I would be pretty darn hesitant to use OPUS, NGS Benchmark or other vertical control that was not shown on the plans.
You, being the expert measurer that you are, must be correct. Do it your way, I DON"T CARE!?ÿ You asked for advise, I gave it, and because you don't agree with an opinion that isn't the same as yours, you want to engage in an on-line argument. Since your profile does not indicate if you are licensed or not, I will advise you in this manner.?ÿ If you are licensed, complete sign, & seal the form. If you are not licensed, ask your boss how he/she wants it completed.?ÿ Like I have previously mentioned, just because you have an optional way of establishing an elevation, does not guarantee that the approving authority will accept your methods.
@skeeter1996 well I think opus is fine. It has a tie to navd88 any way. Only time I would not use GPS or OPUS on a elevation certificate is when its to close for geoid and gps accuracy for a client to have to pay flood insurance. Then i would rune levels if possible. The reality is though that many marks have not been re leveled and not all leveled marks have had GPS observations on them. That is a reason for GPS on Benchmarks. With the airborne gravity and studies i have seen the passive marks will not be as important in the new datums. Not because they are inferior but cost to maintain them is to high. ?ÿI love visiting old marks and many are very stable. And have served as a great source for many years many many years. ?ÿIf i were rich beyond my means i would be visiting and keeping them updated as a way to preserve and keep them. But i am not and i wish somehow the local states and or surveyors could preserve them. It is just the federal government is not.?ÿ
The new datum will i am sure have its obstacles but will be designed to possibly eliminate passive marks all together as far as maintaining them. Opus is a tool by NGS and a benchmark that has been published is access to a datum. Opus is access to the same datums as well. ?ÿJust not through that physical monument. It is through indirectly many many monuments over time. They both have pros and cons.?ÿ
@michigan-left I'm puzzled by your post NGS Benchmarks were converted from NAVD29 to NAVD88 by adding 3.7. The only time I've converted NAVD29 benchmarks to NAVD88, I've used NGS's conversion program. It varies sometimes considerably depending on how close you are to the mountains and what your Latitude, Longitude is.
FIRM map accuracy is another subject.
I stand corrected. The FIRM Panels do have NGS Benchmarks shown on them. I usually only retrieve FIRMETTEs from FEMA to locate the nearest cross section to determine the location in the FEMA Flood Insurance Study. FIRM map quality is another subject. The subject is OPUS versus NGS Benchmarks.
How old are you? What's a charlatan. That sounds feminist.
@skeeter1996 it sounds Shakespearean to me. We go to unlevel the datum charlatan!
@dave-karoly Yeah I think you're right. Is he trying to intimidate me with his superior culturedness?
Let's just say I'm old enough to know that you're likely in more trouble than you're probably aware of, and it's likely that you will be the last to know.
You might benefit from the collective wisdom at this site.
I can be a salty SOB, and I apologize for that, but everyone here is trying to help you.
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Thanks for identifying benchmarks in the area.
Note that the first two points (SS0852 and SS0849) have NAVD88 adjusted heights. Their NGVD29 heights are both indicated to be ??ADJ UNCH? which means: Manually entered unverified output from a least squares adjustment of level data.? I would have low confidence is ADJ UNCH heights. Nonetheless subtracting the NAVD88 and NGVD heights for the two PIDs shown yields differences of 4.07 and ?ÿ3.09 (in units of feet).?ÿ
I ran these points through some of the tools shown in my first post on this topic. With only my iPad Pro on hand I am unable to run Phase1.
Using the Project Identifier from PID, I find that the two PIDs were ONLY used in one NGS level project: L18721.
Running the Leveling Project tool yields the following information.?ÿ
From L to R
Project summary and statistics, actual section leveling data (uncorrected and unadjusted), graphics showing location of SS0849, the entire level project and SS0852.?ÿ
The OP-er should read https://www.ngs.noaa.gov/PUBS_LIB/NAVD88/navd88report.htm ?ÿfor an better understanding of the shift to NAVD88 from NGVD29.
Note that the ??List Observations? option shown in the graphic shows section distances as well as height differences. It also shows both forward and backward runs as well as reobservations.
When I return, I will run the Phase1 tool. It??s on my checklist??
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@rover83 I believe the confusion is the ambiguity restring the term ??ortho??. Ortho just means corrected, so where you are talking about orthometric heights, the gentleman failing to understand may be thinking you are?ÿrefering to an ortho photo.?ÿ
FWIW, another way to long post.
Using the two BM datasheets (copied below) and the Leveling Project Page information (my last post) we can compare the field height differences with the differences between published NAVD88 heights on the datasheets.
In the event anyone has difficulty following I extract data and reorganize.
Rather than using either the PID or designation, the Leveling Observation adds a SSN (station serial number). In this case, SS0849 and SS0852 are SSN 0171 and 0174 respectively.
Observed section differences are (in meters):
from to forward run ?ÿbackward run ?ÿmean de
171 -> 172 -2.524 +2.52270 -2.52335
172 -> 173 1.92270 -1.92320 +1.92295
173 -> 174 -2.20400 +2.20100 -2.20250
Summation of differences in elevation from 171 to 174 = -2.8029
Published NAVD88 heights 171 = 1054.781 for 174 = 1051.977 difference = 2.804 (agrees with field observations by 1 millimeter.
The published NGVD29 difference between 171 and 174 = 2.808
?ÿI like to use the field observed height differences rather than mere differences in published values as they are not impacted by the choice of constraints in the adjustment. As the field differences do not include post-observation corrections they will directly comparable to your field differences.
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The objective here should not be trying to determine what is the most correct, true, accurate or precise NAVD88 elevation. The objective should be trying to MATCH THE DATA THAT WAS USED TO BUILD THE MODEL that established the flood elevation reported on the FIRM map.
Imagine an architect designs a building site and based all his plans on the "fact" that the GIS map showed the 796 foot contour following the top of curb of an small island in the road in front of the building. He calls that his benchmark. A local site map is created and the building is constructed using this "benchmark" of 796.00 and let's say the first floor elevation is 800.00
... Several years later the same architect draws up plans for an addition to the building and he calls for the first floor elevation of the addition to be the same 800.00 as the original building. The original building had frost footings, but the new addition is going to have a basement, so the footings won't be at the same level. The concrete guys can't just extend the footings at the same level, so they call up Surveyor Joe to stake the footings for them. Surveyor Joe shows up and he uses his state of the art GNSS and a long observation of the top of curb on the island gives him an elevation of 796.82 NAVD88. Previous work in the area showed an elevation for the top of curb to be 794.37 NGVD29, which using NGS VERTCON for his location gives him a top of curb elevation of 795.73 NAVD88. The numbers don't match, so Surveyor Joe looks a little closer, the curb doesn't look like it has been replaced or even disturbed, but maybe it has, you never know....
If he uses any of the above measurements to stake the footings, the building is not going to line up right. So what number should he use?
It does not matter what the "TRUE" elevation the curb is today or not. It doesn't matter if he has the most accurate number from the greatest source available, or just a number scribbled on the back of a gum wrapper.
His job is to MATCH THE DESIGN.
In the above case, Surveyor Joe should measure the first floor and compute elevations to stake the footings based on that, regardless what elevation that would make the curb compute to be.
There may not be a monument at the 100 year flood to measure and compute from, but there is a LOT more data about the model used to compute the flood elevation than is found on a firmette. Firmettes only show a small portion of a single full map panel. If you are lucky, the full map panel may even show a benchmark and the elevation used in the model. If not, you may need to dig a little deeper. The Flood Insurance Study Report (FIS) is a multi page document which contains a wealth of data about how the model for that flood study was prepared. For example, the FIS for my home county explains how several sources of data were used. These included some areas being based on Quadrangle maps referenced to NGVD29 as well as other more local studies (some recent and some older, based on different datums) and the exact method FEMA used to convert this data to NAVD88. There is also data for cross sections, profiles, etc. Certain reports even list the company that provided the control for FEMA's work, so you could contact them directly regarding if they were using NGS benchmarks vs OPUS, or network RTK, or ???.
Also note, if the FEMA model is based on a quadrangle map, there may be metadata for the quadrangle that helps you.
By all means, please, I highly suggest showing any significant differences you find between OPUS observations and NGS published benchmarks (it helps to alert others of possible issues they might overlook) but the ultimate goal of floodplain work is to match the data that was used to build the model that established the flood elevation reported on the firm map.
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Bonus tip -
It is also worth looking at the full FIS in some of those situations where you are very close to the flood elevation shown on the map panel or firmette, as those typically show flood elevations to the nearest foot rounded up, but the study will often show values to the nearest tenth of a foot, and that can be that be the make or break of a building being in the floodplain or not
I do not see anything that says specifically if you are licensed or not or what your experience level is, so don't take any of this personally
First off, I like the fact you are doing TWO things right,
1) CHECKING your work
2) ASKING QUESTIONS when you are not sure
In my experience (which I grant you is not in a mountainous region subject to more rapid slide, uplift, rebound, etc.) my GNSS observations usually agree within 0.1' or better of NGS benchmark values (for undisturbed monuments)
That is for benchmarks with 1st or 2nd order elevations. 3rd order can still be plenty accurate, but it really depends on what the datasheet says.
(If you are unfamiliar with 1st order vs 3rd order, etc. the following gives an overview)
https://www.ngs.noaa.gov/FGCS/tech_pub/1984-stds-specs-geodetic-control-networks.pdf
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A quick search of Craig, Montana monuments yielded the following:
https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=SS1684
Highlights of this datasheet:
DESIGNATION - CRAIG
PID - SS1684
*CURRENT SURVEY CONTROL
______________________________________________________________________
* NAD 83(1992) POSITION- 47 08 10.91742(N) 111 57 47.96144(W) ADJUSTED
* NAVD 88 ORTHO HEIGHT - 1310.6 (meters) 4300. (feet) VERTCON
______________________________________________________________________
HORZ ORDER - THIRD
.The NAVD 88 height was computed by applying the VERTCON shift value to
.the NGVD 29 height (displayed under SUPERSEDED SURVEY CONTROL.)
.Significant digits in the geoid height do not necessarily reflect accuracy.
SUPERSEDED SURVEY CONTROL
NAD 83(1986)- 47 08 10.92197(N) 111 57 47.92313(W) AD( ) 3
MORC - 47 08 08.17000(N) 111 57 39.28000(W) AD( ) 3
NGVD 29 (06/04/92) 1309.7 (m) 4297. (f) VERT ANG
.Superseded values are not recommended for survey control.
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Even tho it lists a NAVD88 ortho height, and was computed using the awesome VERTCON program, in the end it is essentially just a rough estimated value. The computed value is rounded from a conversion of an already rounded NGVD29 value which was obtained from a vertical angle (from an unspecified source) and NOT from conventional leveling.
This kind of double rounding of an estimated value to begin with would be lead to exactly the type of situation you describe where sometimes you are commonly off a foot, sometimes half a foot, etc.
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