Activity Feed › Discussion Forums › Strictly Surveying › Would someone check my math please?
-
Would someone check my math please?
Posted by rfc on February 9, 2017 at 1:18 amI’m adjusting all the bubble vials on my tribrachs.I’ve done all the circular ones in the base of the tribrachs, and now doing the vials in the Topcon prism adaptors (that rotate in the tribrach). I’m having a heck of a time getting one precisely adjusted, even having moved it from one to another tribrach to check it.
It’s within one line on the vial, which states “30” per division”, so I’m assuming if the edge of the bubble is touching the first line to one side of the two bubble centering lines, it’s (potentially) out of plumb by 30″.Therefore, if my prism center is say .6′ above the rotation point at the tribrach screw level, then the horizontal error of 30″ = .6 x Tan (.00833 degrees), or .00009′ offset.
I can move the bubble the one division without barely seeing the circular vial move (about perhaps 10 or 20 degrees of leveling screw turn.
So the question is: If my math is correct, why am I worried about this at all? Seems like I might have bigger fish to fry than this one.
Kris Morgan replied 7 years, 6 months ago 4 Members · 11 Replies -
11 Replies
-
I think the math is ok. But the distance you should be calculating with is the height above the ground mark. Even there, a few arc minutes aren’t a big deal.
. -
Bill93, post: 413172, member: 87 wrote: I think the math is ok. But the distance you should be calculating with is the height above the ground mark. Even there, a few arc minutes aren’t a big deal.
Yup, I see your point
Bill93, post: 413172, member: 87 wrote: I think the math is ok. But the distance you should be calculating with is the height above the ground mark. Even there, a few arc minutes aren’t a big deal.
Yup. I see your point. If I’m using the optical plummet in the prism adaptor, that’s pointing at the mark. At 30″ and a 5′ height, that ups the error to .00073′.
I usually split the difference if I can’t get a vial like this level…I go back half the distance, then turn it 180 and see if it’s the same. -
rfc, post: 413164, member: 8882 wrote: I’m adjusting all the bubble vials on my tribrachs.I’ve done all the circular ones in the base of the tribrachs, and now doing the vials in the Topcon prism adaptors (that rotate in the tribrach). I’m having a heck of a time getting one precisely adjusted, even having moved it from one to another tribrach to check it.
It’s within one line on the vial, which states “30” per division”, so I’m assuming if the edge of the bubble is touching the first line to one side of the two bubble centering lines, it’s (potentially) out of plumb by 30″.Therefore, if my prism center is say .6′ above the rotation point at the tribrach screw level, then the horizontal error of 30″ = .6 x Tan (.00833 degrees), or .00009′ offset.
I can move the bubble the one division without barely seeing the circular vial move (about perhaps 10 or 20 degrees of leveling screw turn.
So the question is: If my math is correct, why am I worried about this at all? Seems like I might have bigger fish to fry than this one.
Are you sure if the procedure to adjust a vial?
I reality, a vial is always correct. If it’s 1.5 marks to left, and you rotate it, and it’s 2.5 marks to the right, then center is 2.0 marks. And rotate it, it stays at 2.0 marks, done. Then center the bull’s eye.
My T3 is like that. It’s not worth adjusting.
-
Larry Scott, post: 413372, member: 8766 wrote: Are you sure if the procedure to adjust a vial?
Pretty sure. Once the tribrach is level (using the total station’s compensation screens to get it level), I put the prism adapter on; level the vial; rotate 180 degrees; If it’s off, go back half way; rotate again etc. until it’s centered. Then turn it to all four quadrants to check it. This one is a pretty crappy used one I got on eBay, so I don’t have great expectations, but I’ve found that well adjusted crappy equipment is usually better than mal adjusted good stuff lol.
-
There’s your problem.
Level up using total station. Put your prism in the already leveled tribrach. The vial’s bubble might be eccentric, but shouldn’t move when rotated, if the tribrach is level per total station.So if you level with the TS, it’s level. Don’t relevel the prism, adjust the prism vial.
But that’s when spirit leveled. By the vials. I don’t know about all that fancy e-lectronuc compoundsated stuff.
-
Larry Scott, post: 413376, member: 8766 wrote: There’s your problem.
Level up using total station. Put your prism in the already leveled tribrach. The vial’s bubble might be eccentric, but shouldn’t move when rotated, if the tribrach is level per total station.So if you level with the TS, it’s level. Don’t relevel the prism, adjust the prism vial.
But that’s when spirit leveled. By the vials. I don’t know about all that fancy e-lectronuc compoundsated stuff.
I’m only using the total station for calibration. In the field, I will often set up a prism on a tribrach without first having put the total station upon it.
If the bubble seems off (as it has been for this particular one), I “average the difference” in all directions. I just think this one is a piece of do-do.Fortunately, the resulting errors appear to be small, although I’m glad I’ve quantified them.
-
Put it in a tribrach, get it close, spin it 180, see the difference, take out 1/2 the difference, go back 180, see the difference, take out 1/2 the difference, rinse and repeat and THEN adjust the bullseye bubble since you can’t spin it.
That’s what the books taught us.
-
Kris Morgan, post: 413411, member: 29 wrote: Put it in a tribrach, get it close, spin it 180, see the difference, take out 1/2 the difference, go back 180, see the difference, take out 1/2 the difference, rinse and repeat and THEN adjust the bullseye bubble since you can’t spin it.
That’s what the books taught us.
That sounds fine, but when you say “take out half the distance”, you’re referring to taking it out by adjusting the tribrach? That seems like it would not work, as there’s no guarantee that the vial on the prism holder is parallel to the interface between it and the tribrach. I’ve got two bubbles to adjust. I don’t see how you can start at the top and work down.
If you mean take out half the distance by adjusting the vial itself and you do this first, there’s no guarantee that the tribrach itself is level.
-
Your first goal is to set the tribrach leveling screws so the instrument bubble, or a tribrach adapter with its own bubble, doesn’t move (stays on same line if not centered) when you turn the instrument. To get close, use the leveling screws to put the bubble halfway between the positions you see before and after a 180 degree rotation. You won’t necessarily be be going halfway to the center of the vial. Repeat.
Then the instrument, tribrach adapter, and tribrach are level and you can set their bubbles to center.
If it was a long way out of adjustment, the first pass may not get it exactly right so you repeat the whole procedure.
The next step is adjusting the optical plummet to point to the spot directly under the leveled tribrach or instrument. There are various techniques that have been discussed in a recent thread. I use the pencil line on tripod head, rotate tribrach to 3 positions, and figure I can get within a millimeter that way if careful.
. -
to put a tribrach over a point, I find using an instrument 12-15 feet away and lock on the surface mark. Then transit the scope to the target. Then move the tribrach as necessary. I set 2 instruments, (T1 and a T16) 90å¡ apart. Then when the tribrach is level and perfectly over the mark – by transiting the instrument, I adjust the plummet. And of course, do that a few times.
-
rfc, post: 413467, member: 8882 wrote: That sounds fine, but when you say “take out half the distance”, you’re referring to taking it out by adjusting the tribrach? That seems like it would not work, as there’s no guarantee that the vial on the prism holder is parallel to the interface between it and the tribrach. I’ve got two bubbles to adjust. I don’t see how you can start at the top and work down.
If you mean take out half the distance by adjusting the vial itself and you do this first, there’s no guarantee that the tribrach itself is level.
If done properly, the tribrach HAS TO BE level when you’re completed as you can spin it and the plate vial bubble always reads level. THEN you adjust the bullseye bubble and optical plummet, if necessary.
Log in to reply.