I noted the reference Kent made to "forced centering" in Yswami's now epic 100+ post thread, and a search turned up this one:
https://surveyorconnect.com/index.php?mode=thread&id=191125#p191219
In my own learning voyage, I've been stumbling towards "forced centering" already, without even knowing what it was. I haven't always set the instrument up at the forward station, but I have three tripods, three identical prisms (that I've two peg tested multiple times), and this winter have modified them to be precisely as high as the instrument. Preparing for the next round, I see much discussion about making sure the tribrachs are in good adjustment, so want to check that off my list of "to do's".
What is the best way to check, and adjust, a tribrach?
A relatively crude field check is to level up and center over a point, then, with a pencil, trace the outline of the tribrach base on the tripod head. Then loosen and turn the tribrach 120° and center it exactly inside your tracing. Tighten and level it up using the tribrach footscrews only. Once level you should be exactly on the point again (in practice you will be within a hundredth or so with well adjusted tribrach).
That was a good rough check. At my last firm I calibrated all the company tribrachs, usually on a quarterly basis. Monthly if we were doing a lot of precise layout work.
Procedure for calibrating the tribrach.
Set a tripod (usually with three drilled hockey pucks chained to a large steel ring to hold the legs stable) in a place where you have at least a 10 ft high ceiling.
Set first tribrach on tripod and level bubble. Then set your most recently calibrated theodolite or total station on tribrach and level to the instrument bubble. Then adjust tribrach bubble to match instrument level, adjusting the bubble level screws.
Remove instrument and place 2nd tribrach upside down on top of the calibration block (a round cylinder (2 in. tall) of metal or derlin with parallel machined ends) that fits inside the 1st tribrach. Center 2nd tribrach cross hairs on a small point, using 1st tribrach leveling screws. Focus cross hairs of second tribrach on the small point on the ceiling. Rotate 2nd tribrach 180 degrees and note where the cross hairs move to. Adjust cross hairs to half the distance moved. re-center 2nd tribrach to the small point on ceiling and check again and adjust until rotating 2nd tribrach cross hair stay centered. Swap 1st for second and repeat procedure until all tribrachs are calibrated. We kept an old Wild T2, service dept. adjusted, (never to be used in the field) just for calibrating the tribrachs