A while back I posted about a bench mark that is on the NGS priority list for GPSonBM, but is in a poor signal environment next to a building and flagpole.
https://surveyorconnect.com/community/gnss-geodesy/blunder-and-or-gps-site-unusable
It gave me a really sloppy OPUS session, 48 mm pk-pk.?ÿ I broke it into three 2-hour sessions, and the 2nd and 3rd agree with each other within 25 mm, but the first is pretty wild and different.
Yesterday I went back and did a 4.6-hour session on an offset point with better sky, and leveled (redundantly) from the antenna ARP to the bench mark.?ÿ The rapid-orbit solution is tight (6 mm pk-pk El ht) and the old 2nd and 3rd short sessions over the mark agree with it within 15 mm.
What I find is a pretty strong likelihood that the reset calculation was blundered by 1 foot. If I assume the data sheet ortho is 1 ft too low, then (data sheet ortho+1 ft) - (OPUS-derived ortho) difference is within a cm of what I find on a couple stations 20 miles away (difference being geoid error+GPS error).
I'll probably turn in an NGS recovery report saying that 2 sessions find the mark about one foot higher than the data sheet.
I may go to the County Engineer's office and ask if they have the field book for the 1980 reset operation.?ÿ Their office would be likely to have done the reset.?ÿ They will consider me a bother, and won't care about this NGS mark because they have a 3-mile grid of GPS points that doesn't include this mark.?ÿ But I'd like to see the raw data.?ÿ Is this worth pursuing?
I could do an OPUS Share of my offset session (when precise orbits come), but that won't have a PID and be noticed by people who look at the regular data sheet.?ÿ
I could share a 4-hour portion of my session over the disk that gives a similar elevation to the offset measurement so it is tied to the PID.?ÿ But I wouldn't want NGS to use that elevation to check or fit a geoid.?ÿ And I may have to play with choosing CORS to fudge that result to match.
Any recommendations on how to proceed?
Bill,
Reset procedures that only involve the mark being destroyed are third order. They only get published to the tenth and are not checked for subsidence or rebound.
If you want to upgrade it you need to follow the processes outlined in the NGS Benchmark Reset manual (on the NGS site). Alternatively you can submit a recovery note mentioning the bust (after you recheck yourself) and see if the note makes it onto the sheet.
The original was destroyed in 1981, so if the transfer can be checked it is only by finding the person's field notes.?ÿ The only way to upgrade, and maintain connection to NAVD88 as originally leveled, would be to run good level lines for many miles to existing marks.?ÿ But it still has poor sky, so why bother trying to improve its order? Fixing a blunder might be worthwhile, though.
I think I have rechecked myself, as I have the consecutive 2-hour sessions and an independent 4-hour session.?ÿ Even as wild as that first 2 hours was, all of my sessions and some alternate choices of CORS all come out at least 3/4 foot above the data sheet elevation.
NGS must have picked this (3rd order, poor sky) mark for their priority list because there is no other one within 20 miles reported as found and with better sky visibility.?ÿ There is a big hole in the GPSonBM coverage from here northward.?ÿ
If NGS allowed the use of offset positions and a one-turn level run for GPSonBM, there would be more choices.?ÿ That county did a GPS grid in 2003 and used a few NGS stations by running levels to clear sky positions.
If you share your OPUS solution the NGS has the option to use it in some of what they are doing. They do not currently use any OPUS submissions that arent shared.
One thing I would do is look for any other types of control nearby (BOR, Power companies, etc.) purporting to be on a known datum. The idea is to determine for yourself if there is a bust or if it's a subsidence issue.?ÿ
Good on you for trying, but bluebooking and a 20 mile 1st or 2nd order double run is not in the budget for most of us...?ÿ
?ÿ
The priority list selection has a few head scratchers on it. The selected mark in Mullan, ID is a vertical mark iirc- and one I found destroyed last week T 74 was recovered in 1970 and listed as poor condition, having been hit and being loose and out of plumb. I believe you??re correct in that they ended up picking marks based on location- but they could have been a bit more thorough. Some I think are just wishful thinking.
Yes, when you find their list of 73 marks in Iowa to contain a vertically mounted disk, marks listed as NF in 1969, 1980, and 2000,?ÿ one with a DOT shared solution (that I confirmed later) showing it is most of a foot off, and several that have photos showing they don't have good sky, you know they didn't do a lot of study of the available data when choosing.
In their defense, it's a huge job that can't be fully automated to choose 5800 marks across the country out of the hundreds of thousands of data sheets, while considering coverage, prior coverage (e.g. Geoid12B stations), suitability, recovery status, stability, etc.
