Tribrach Calibration

A quick Calibration would be to use a plumb bob, mark where it hits the ground, and see if your laser hits the mark you made.

Thanks Scott!

I'm sure you saw the [msg=155602]old thread[/msg] that was recently revived on tribrach calibration. The fact that it uses a laser pointer instead of you looking through the optics should cause no difference in procedure.

> I'm sure you saw the old thread that was recently revived on tribrach calibration. The fact that it uses a laser pointer instead of you looking through the optics should cause no difference in procedure.

Aloha, Bill: I did follow the thread. I didn't see the adjustment screws hence the question. I just took a look at the tribrach it has the adjustment screw but tucked slightly under the tribrach itself. Thank you so much for pointing this out!!

> A quick Calibration would be to use a plumb bob, mark where it hits the ground, and see if your laser hits the mark you made.
That method is indeed very quick, and nearly foolproof, but not very precise. I doubt you could detect 0.01' that way.

Here is an alternate, somewhat more precise (although not as precise as the tribrach donut method) way to check a tribrach. Set up the tribrach, level, and center over the point. With a pencil, trace the outline of the tribrach on the head of the tripod. Loosen the tribrach, rotate it 120°, center the tribrach in your pencil tracing, and tighten the clamping screw. Re-level the tribrach using the tribrach screws only. If your tribrach is in good adjustment you should be on the point again exactly. The precision of this method is limited by your ability to get the tribrach exactly inside your pencil tracing.

But I think that there is something else going on, unrelated to the tribrach centering. I know that you are using a Trimble 5603, an instrument I have had a good deal of experience with. Running the compensation routine, no matter how bad the adjustment of the tribrach may be, should have absolutely no effect on the tribrach cross hairs. None.

It is likely that the level bubble on the tribrach is off. That would cause you to have to have to adjust the tribrach screws quite a bit during the electronic levelling process that preceeds the compensation process, which would cause the cross hairs to be moved of the point. Next time you perform the instrument set up look at the tribrach level bubble after you have completed the compensation. If the bubble is off center that is your problem. Your instrument is level at this point- adjust the tribrach bubble.

It may also be that the tribrach legs are loose and are moving during compensation. But they would have to be very, very loose to cause that much of a difference with that little bit of use.

Most of the laser plummet type tribrachets I have seen do not have any adjustment capability. Once they are out that is it.

Aloha, Norman:

Thank you so much for the detailed reply!

I read about the tracing the outline method. I was questioning my ability to precisely placing it again within the outline. That's why I purchased the tribrach "donut." Then I don't see the adjustment screws...which I think I found now.

You are correct I have 5603. I'll do the test and will report back. It sure makes a lot of sense once I read your explanation above.

I think, I will check the level bubble first before making any adjustment to the laser plummet--if it is adjustable.

Thanks again!

> Most of the laser plummet type tribrachets I have seen do not have any adjustment capability. Once they are out that is it.

Aloha, John:

That's what I thought...but I see two screws. I will adjust them after checking the level bubble per Norman's suggestion above.

Thanks!