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NGS Benchmarks versus OPUS derived Elevations
Skeeter1996 replied 1 year, 8 months ago 21 Members · 54 Replies
@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 refering to an ortho photo.
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 backward run mean 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
I 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.
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.
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 notI 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 sureIn 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
A quick search of Craig, Montana monuments yielded the following:
https://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=SS1684Highlights 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.
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.
- Posted by: @reddog
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.
This.
. - Posted by: @bill93Posted by: @reddog
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.
This.
Double this ^^^^.
While FEMA has updated the mapping of the flood plain lines, the model used to define those lines could still be based on surveys run before GPS elevations were available.
I worked on a flood plain study for FEMA in 1990 after a major flood event in Maine. We only measured cross-sections at the bridges and culverts that had been destroyed and replaced in the prior flood. All the cross-sections at the “undamaged” constriction points were re-used for the new flood model.
NC Flood Inundation Mapping (FIMAN)
Quality mapping saves lives, saves money, draws businesses, and in general make it easier to make good decisions. This entire thread is another indictment of our inability as a country to make wise and long term decisions. The tools needed to generate the real time map linked above also provide free statewide orthometric imagery, free LiDAR that can be immensely useful for planning, and over time provide snapshots of the past.
FIMAN has predictive capabilities, the accuracy of which is astounding and have been verified numerous times by expert geodesists as well as anecdotally by PLSs and visual observance of high water marks.
It took back to back hurricanes to prompt North Carolina into action, but we quickly realized that its value far exceeded the cost. What will it take in your state?
That’s some pretty fancy stuff.
It would be nice if all federal, state, and local gov’t were at that level of accessibility and usefulness, etc.
I try to have my elevations match the RM(s) on the FIRM. Put it back on FEMA.
I think it??s only fitting that we have vertical pincushions if we have horizontal pincushions. I would base my elevations on the recovered NGS Benchmarks.
Shown below are the input file for Phase 1, Output for two points in previous post, and header information for output file showing corrections applied. In this run all possible corrections were applied. Corrections are listed in the header section.
BTW, there is another NGS tool for transforming published NAVD88 orthometric height differences into the leveled height difference by removing the orthometric correction.
Extracted from the tool’s documentation:
I received a PM asking me to provide information about a number of monuments. While willing to point to resources and provide examples of the tools available, I do not provide additional free services. Nor am I interested in taking on fee-based services.
To expand and elaborate on information I provide the following.
To find a PID given only a designation, one uses the following link. You enter the designation, select the state in which its located and submit. Note that the date is not generally part of the designation.
https://geodesy.noaa.gov/cgi-bin/ds_desig.prl
Once you know a PID, use this tool to find projects associated with it: https://www.ngs.noaa.gov/cgi-bin/SURV_ID/mark_sources.prl
L 435 is PID SS1269 the only level project in which it appears is L24479/6 (the /6 means that it is part 6 of L24479).
K 455 is PID RX0124 there are two level projects associated with this BM. A line observed in 1936 is L7684/5; the BM also appears in a 1961/2 level line L18718/3.
Using the Leveling Project Page https://beta.ngs.noaa.gov/datasheets/leveling-projects/index.html you can display and retrieve a multitude of information after entering the level project designation.
See previous posts for screen captures of the Phase 1 output for L18721. See previous posts for a link to the Phase1 tool. The complete output file is shown below:
Note that this is output where all corrections were applied.
@kevin-hines I was gonna say, the instructions are rather plain on using OPUS derived elevations.
@joe-the-surveyor
FEMA accepted the OPUS Elevations and issued a LOMA taking the area out of Zone A and making it Zone X. The County Flood Plain Administrator dug up the sewer line topography, compared them. They differed by a little over a foot. No benchmark used indicated on the Engineer’s topography. The County objected to FEMA, and they rescinded the LOMA.
FEMA agrees with OPUS, it’s the County Flood Plain Manager that’s the problem. FEMA replied it’s the local officials call. But FEMA audits the hell out of the local governments checking their Flood Plain decision, but it appears to me doesn’t give them any training. Appears to me the Counties should be required to have a reviewing professional.
Making accusation of fraud over a foot difference in Flood Plain arguments is utterly ridiculous in my opinion.
5 different Engineering firms, plus FEMA did Flood Studies and none of the BFE’s were the same. FEMA’s BFE differed by 5 feet. How do you make anything match existing data.
I feel OPUS gives some credibility to the Elevations stated. Comparing Flood Plain elevations to property surveys is ridiculous. They are totally not related, but the pin cushion theory is definitely related.
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