I've been experiencing several instants of NGS benchmarks not matching each other's published elevations. I know that Highway R/W markers are put in after the survey. Is it possible the benchmark monumentation is placed after the surveying is done? Has anyone gotten copies of the actual notes from NGS for a specific level loop?
Curious minds want t know.
What kind of differences are you seeing?
Not a chance
Is it possible the benchmark monumentation is placed after the surveying is done?
I got a chuckle out that, too.
@jim-frame I know the Highway R/W concrete markers are put in after the survey. The State Surveyor says he knows they are off, to find the reference rebar nearby that is the surveyed R/W. Trouble is the Right of Way Plat calls the Concrete Monument. NGS benchmarks match OPUS about the same 1 foot different.
In my area most of the benchmark levelling was done long ago. The 1930's were a big time in the benchmark levelling business. Since then a lot has happened, geospatially speaking. Subsidence is a big thing in Louisiana.
Is it possible the benchmark monumentation is placed after the surveying is done?
Do you mean set them as a side shot from the level loop?
Subsidence is not a problem here. I don't know what the closure tolerances were back then, but I bet they weren't 0.0 feet. I'm betting they were closer to a 1.0 foot.
That's why I was wondering if anyone had ever seen a NGS level loop notes or closure report.
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What do elevation benchmarks have to do with road monuments??ÿ
Well it was the process of setting them. NGS benchmarks are usually on concrete columns 1 foot or more high. Did a crew set the monuments first and then an elevation was established on them after they were constructed or was an Reference point set and the Loop elevation transferred to the monument. Highway R/W monuments are concrete columns similar to NGS benchmarks and I know for a fact Highway R/W monuments were set post survey off of a reference pin by a Contractor. Did NGS set the benchmarks and the Leveling crew survey into them or were the benchmark monuments set later by a Monument crew or a Contractor from References left by the Leveling crew?
That's the relevance.
I know that when Curt Smith was the NGS advisor here, he provided notes/data to The Helena crew because the were trying to isolate some errors. I know if I don??t match an NGS bench, I check my stuff first. Sometime the NGS are off, but it??s usually the older caps on concrete moms. Driven rods are seldom a problem.?ÿ
Thanks for the input. The concrete monuments are the most problem.
Super good reference. That answers my question if the concrete monuments were set after the survey.
Thanks much!
If I was off by one foot vertical on an NGS elevation BM in NC I'd first assume that I did something wrong.?ÿ For that matter, any level run that is off by a foot is a sure sign that you misread the rod.?ÿ Look through your notes and find places where the backsight or foresight was in the upper tenths range of a foot.?ÿ You probably read something like 5.95' as 6.95'.?ÿ I'm not a fan of the metric system, but I'll admit that in leveling projects, reading centimeters eliminates the one foot bust.
There's a ton of info to unpack in any NGS data sheet.?ÿ?ÿ
GNSS has taken most of the fun out of finding NGS BMs.?ÿ I was instructed early on to print out the data sheet, then highlight the pertinent information such as horizontal or vertical order, ortho height, SPCS coordinates, last recovery date etc..?ÿ I remember a particular time I didn't do this and wasted a ton of time looking for a BM that had scaled horizontal coordinates.?ÿ I found it, then noticed it was useless to me when I began entering the coordinates in the DC.?ÿ
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C&CS Initial tolerances were in millimeters and carefully checked.
Some poured in place concrete posts were not as deep as they were supposed to be and in areas of frost have moved. Subsidence areas have seen big changes. A few instances have been reported where monuments were disturbed and put back "where it used to be". RESET monuments occasionally had blunders- I found one that I think had a 1.00 foot blunder in the reset.
MathTeacher??s ??Scope of Work? document includes links to NGS manuals. I suggest you read NOAA Manual NOS NGS 1 ??Geodetic Bench Marks? (1978) which clarifies how NGS approaches monument setting.
FWIW, NGS does not ??usually? set monuments protruding above the ground surface. Highway right-of-way markers are not established by NGS nor would they be used by NGS as monuments as they are too susceptible to disturbance.
As for NGS leveling observation records, use the ??Leveling Projects Page? at ?ÿ https://beta.ngs.noaa.gov/datasheets/leveling-projects/index.html
The observations shown are abstracted section records. While techniques have changed over the years, modern leveling requires sections of leveling to include forward and backward runs which must agree within standards or be rerun.
Users retrieve information using the level project number. If you have a PID you can find the project ID via a link on the linked page above.
??Screen captures follow:
What are those standards? Are they the same or vary by area?
see section 5 of MathTeacher??s SOW document for links to specification documents. Note that the leveling specs are nationwide in scope and ?? authorized? by the Federal Geodetic Control Subcommittee.
The Federal Geodetic Control Subcommittee helps coordinate the planning and execution of geodetic surveys, developing standards and specifications for these surveys, and exchanging geodetic survey data and technical information among Federal agencies. Description from the FGCS home page.
The leveling specs are here:
https://www.ngs.noaa.gov/FGCS/tech_pub/Fgcsvert.v41.specs.pdf
While many people seem to think all benchmarks are established by NGS numerous other entities have and continue to establish benchmarks. Much of this work is NEVER submitted to NGS.
One argument for shifting to the new geopotential datum is that monuments, once established, are infrequently or never checked/verified by new leveling. All monuments are subject to disturbance.
If you have verified your observations by forward and backward runs, verified your equipment to be in good order and adjustment, insured that no monuments are misidentified (e.g. observing to a reference mark instead of the station) and believe that you have identified an area where published heights for NGS monuments are in error or suspect, contact your NGS regional advisor.?ÿ
I noticed a reply to this thread mentioning ??side shots.? If this means an extra FS to a monument, NGS does not do it. Reference marks clustered near the actual mark are included in the leveling.?ÿ
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