Blundered reset?
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. 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. 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. Their office would be likely to have done the reset. 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. But I’d like to see the raw data. 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.
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. But I wouldn’t want NGS to use that elevation to check or fit a geoid. And I may have to play with choosing CORS to fudge that result to match.
Any recommendations on how to proceed?
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