GIAA Mailbag: On Optical Total Stations
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?
A: When surveyors used transits, a basic test and adjustment that they did frequently was called the height of standards adjustment. You should still be doing this test with your total station, but the adjustment is one that should be taken care of at a qualified service center. When the height of standards is not in adjustment, it means that the horizontal axis of the instrument is not perpendicular to the vertical (or direction of gravity). Raising or lowering the telescope then causes the cross hairs to travel along an inclined plane rather than the vertical plane the instrument’s designers intended. Small errors in leveling the instrument (errors at right angles to the line of sight) can also cause this error to appear. Depending on the source of this error and the built-in mechanical, electronic and software features, the changes in horizontal angle may be attributable to corrections being automatically made. Errors due to the height of standards being out of adjustment are frequently compensated for with software that takes into account collimation errors (if observations to determine the errors are required to be made at large vertical angles). Errors due to leveling are compensated for by so-called “dual axis” compensators, which detect tilt in the direction at right angles to the line of sight of the instrument.
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