I'm hoping to submit a GPSonBM session on a bench mark that poses a bit of a problem.?ÿ It is mounted on the base of a cannon in the city park, and the cannon overhangs almost exactly half the disk.?ÿ No room for a pole and hard to get a tribrach over it.?ÿ
So I made myself a rig to hold the antenna exactly 0.500 meter above the disk.?ÿ A hole in the bottom bracket lets me see that it is centered, and a hole in the top bracket fits a 5/8 bolt to hold the antenna. I plumbed it with a hand level, and expect to be within a couple mm sideways. The board on the tripod is tapped 5/8-11. The braces to the tripod were adequate on a still day, but probably wouldn't be good in wind.
This is where you unscrew the prism and set it right on the cap.?ÿ ?????ÿ
That is determination.?ÿ
Ya but ... I wasn't using a prism. If using a TS a prism would be a nuisance to measure up for vertical but would be fairly good for horizontal position. But horizontal isn't established on this mark, and NGS will code my value as HH1 accuracy, pretty loose.
The purpose here is getting GPS height for comparison with the leveled value and geoid, as part of the 2022 model effort.
Looks good to me except how good is the benchmark concerning stability??ÿ Without a solid underground structure one would think a big bock of concrete with a cannon on it might subside or heave as the decades pass, especially in a grassy soil situation as the photo implies.
Personally I hold ancient benchmarks set in bedrock or the deep NGS unconstrained rod system setup as golden and marks on abutments, in loose soil, etc. as suspect until proven.?ÿ Of course for local control the nearest historical benchmark controls, unless the contrary can be shown.?ÿ But nowadays any GPS derived elevation works locally and nobody cares if you're not measuring from mean sea level, only that the project drains properly and you get paid.
I like it!
Looks good to me except how good is the benchmark concerning stability??ÿ
That's something NGS will evaluate once the process Bill's data.
The setup works for me!
Looks like there's no bolts on the feet of the cannon - is something else holding it in place (and preventing it from being aimed down so that a tribrach could go under it)?
Bill = rockstar!
It is missing one bolt, but has others so isn't going anywhere.?ÿ It does not rotate up or down easily, and I didn't try to force it. I suspect, but didn't check, it may be welded to prevent kids from smashing fingers by moving it.
@mike-marks how about the side of a true cinder block structure on a slab?
