I've been reading the various old threads on tribrach level vial adjustments. On a Topcon target with optical plummet, a vial level on the rotating piece, and a circular vial on the tribrach itself, in what order should one adjust? I'm pretty sure the plummet adjustment is completely independent of the levels, so that wouldn't matter if it were done before or after, but what about the vial vs. the circular?
rfc, post: 346684, member: 8882 wrote: I'm pretty sure the plummet adjustment is completely independent of the levels
May be I am misunderstanding this statement but the plummet is very dependent on accurate level. If I were red-necking an adjustment I would do the circular first, then plummet, then vial, checking all previous adjustments as I go.
There is a tool for this task but it also requires another tribrach in perfect adjustment to use it. See this web page; http://www.pobonline.com/articles/91588-giaa-mailbag-tool-tips-for-optical-plummet-tribrach-errors-part-2
Yes, I've seen the process of using two tribrachs (one upside down on the other), but none of those docs (that I've read) say whether it matters to do one before the other.
As far as I know, the adjustments for the optical plummet align the reticle such that it is concentric to and perpendicular to the physical three leg "turret". Level doesn't come into it. You could adjust it horizontally with the target on the wall (although it'd be less convenient than doing it with the target on the ceiling).
Once that's done, though, you're certainly correct that that turret MUST be perpendicular to the force of gravity.
Assuming your instrument is properly adjusted.
Level the tribrach, using the vial bubble on an instrument, not the round bubble, Turn 90å¡, recheck, re-level if necessary, doing a full circle.
Proceed to adjust the tribrach bubble. Recheck level with instrument vial.
Set up over a point on the ground. Level and remove instrument and attach target. Do not touch level screws on tribrach.
Check the vial bubble on the target, adjust if necessary, turning 90å¡ through full circle. Remove target turn 120å¡ and do again, repeat for a full rotation.
Now you look though the optical plummet, if off the point, adjust it halfway back. Remove and rotate the target 120å¡ and redo, adjust halfway back, do 2 more times for a full circle. Your last adjustment should be very small or nil.
There is no sense in adjusting an optical plummet if the tribrach is not perfectly level.
Paul in PA
It is best if the tripod head is close to level when you start checking so that you don't have to go to extremes on the tribrach screws. If you have a level in your instrument, it will be more sensitive than anything else you have and should be used for the first step.
Think about what the level is telling you. When the rotating level reads the same at any position, regardless of what it reads, it and the tribrach must be level. That is the critical first step.
The unchanging offset in the bubble is its error. If the reading changes as you rotate, that is due to true mis-leveling and the total movement is twice the tribrach leveling error. (More precisely, the non-verticality of the rotating axis, but we'll assume it's perpendicular to the tribrach.)
Then adjust the bubble(s) so they read level.
Now that you have level to work with, get the optical plummet to point correctly. If you don't have the adjustment fixture mentioned, the best way is probably to set up over a piece of tape and mark where the plummet points. Draw a pencil line on the tripod head around the tribrach. Rotate the tribrach to again match the pencil line. Level up, and mark the paper where it points. Repeat for the third position. The true point is the center of the triangle formed on the paper. Adjust the plummet to point there and repeat.
I always adjust using a ring made for this from SECO, the optical adjustment is completely independant of the level adjustments, in fact the tribrach need not be level using the adjusting ring for the optical portion. Adjust the vial first (much higher precision and accuracy too if you rotate 180 degrees during adjustment), finally adjust the bullseye. I actually use my TS and electronically level the tribrach and then adjust the vial and circle bubbles to that reference.
SHG
I use the SECO adjusting circle with a level vial to check tand adjust the level vial. Can adjust several trilbrach on my desk in the time it takes to adjust one outside on tripod and using an instrument or TS.
Some setups I have a 4ft section of pole atop the tribrach and will take the time to setup a TS or two TS and check the top of the pole for complete accuracy to fine tune the centering and level bubble.
I have noticed many level bubbles are not what they were years ago and can be quite difficult to adjust.
Never thought of the fact that with dual compensation, you have the most accurate level "standard". I'll try that. As for the optical plummet, I've done it both ways (target on the ceiling and target on the floor). Thanks all for the input.
