A: You are correct in ignoring the tribrach bubble. Its sensitivity is much coarser than that of the plate bubble. Assuming your plate bubble is in adjustment, it defeats the purpose of leveling with it if you subsequently adjust the leveling screws to center the circular bubble on the tribrach. People who readjust the circular bubble after leveling with the plate bubble probably don’t realize that all level vials—whether circular or tubular—are not “created equal.” Some manufacturers indicate the sensitivity of a level vial by marking it. You may have noticed that something like “30/2 mm” may be marked on your tubular vial. This indicates that the bubble shifts 2 mm along the level tube when the axis of the vial is tilted by 30 arc seconds. Typical total station vials range in sensitivity from 20 to 40 arc seconds. Unfortunately, it is rare to find circular vials (whether on tribrachs or prism/antenna poles) with a sensitivity marking. Their sensitivity can range from 10 to 60 arc minutes! Thus, you can consistently pick the tubular vial over a circular vial when there is a discrepancy, as long as you know that the one you’re picking is in adjustment.
Q: I’ve noticed that the horizontal angle on my total station changes as I raise or lower the telescope. I’ve checked the horizontal motion clamp and made sure it is properly engaged. Why does this happen?