What method(s) do you use to field check the optical plummet of a tribrach to ensure the plummet is perpendicular to gravity?
How are you field checking the fisheye level bubble of your tribrach?
Center and level the tribrach over a point. Pencil outline the tribrach on the tripod head. Rotate the tribrach 120 degrees and align with the pencil marks. See if still sighting the point. Repeat for another 120 degree turn.
If you mark the point actually seen after each rotation, you can then aim at the center of those 3 points and adjust the level.
Place a bubble you know is good in the tribrach. For example the instruments bubble typically is very sensitive and accurate. Put the instrument in the tribrach, level the instrument and then make the tribrach bubble match.
Set up tripod, tribrach & instrument over a flat-topped wooden hub. Make a pencil mark on the hub to use as your first of 3 points. Center optical plummet over mark and level using the precise spirit level on the instrument. After getting finely levelled, adjust the bulls-eye bubble as needed to center it.
Then, run a pencil along the top of the tripod to trace the outline of the tribrach. Rotate the tribrach and instrument 120 degrees and line up the tribrach with the outline you just made. Level up with the spirit level on the instrument, and look down through the optical plummet. Mark the point the crosshairs of the plummet hit on the hub with the pencil. Repeat the turning of 120 degrees, level up, mark a third point. You will now have a triangle marked on the hub (unless your plummet is already correctly adjusted). Adjust the plummet crosshairs to align with the center of the triangle. You should now have the correct adjustment. To check, turn 120 degrees, align the tribrach with the pencil mark, re-level and check that you are hitting the center of the triangle again.
The method Bill and Peter have described is a good field check, and the one I use - in the field as a check. I've never used it to make adjustments.
As for checking the level bubble, you set up the gun and fine level it. Then check the level bubble. Adjust if necesssary.
Run a pencil mark and it will stay for a long time, so I've found.
IMHO
The best solution is to not have optical plummets in the tribrach, but to use mirror carriers with bubbles and optical plummets built into them. Then field checking is as easy as rotating the carrier.
Why won't hanging a plumb bob from it work?
Exactly why I use these exclusively.
Historic boundaries and conservation efforts.
Oh, it will always work. But it takes longer to set up and is not as precise. Clearly, there is a reason that absolutely no one does it that way anymore.
Do tripods even come with the hangers anymore?
With the exception of my GNSS base (precise post will not fit in the case), I have not owned a tribrach with optical plummet in nearly 30 years. Field checking / adjustment is a snap but I rarely find them out. As an added bonus, without that pesky plummet I can use the automatic HI function on my MS60.
What's a plumb bob?
It's similar to a prune, only juicier, Danny.
Don't need no stinking hanger! Just push the plumb bob string up the center of 5/8" tribrach bolt.....perfect every time 😉. Jp
They do still come with hangers. Just got on a crew chief when i was teaching him how to pegg a level and noticed the plumb bob holder had beeping removed. He i didn’t know what that was for but it’s in the truck lol. I also taught him some stadia.
The old pencil tracing a tribrach works great. I now prefer a puck and do it in the office now but did it with pencil for years.