Home » GIAA Mailbag: On Leveling Instruments and Adjusting Compensators
Q: I have been trained when using a dumpy level to make sure the level bubble is always centered before taking a shot. Yet, when leveling a theodolite I’ve been told to center it, then rotate 180 degrees, observe the shift and move it back halfway, if it isn’t still centered. These two instructions seem to be contradictory. Which is correct?
A: The dumpy level instruction (for those who remember or care) only works when the bubble is in perfect adjustment or when running a line of levels with no sideshots, and the backsight and foresight distances are kept balanced on each setup. With dumpy levels, theodolites, total stations—any instrument that has a tubular vial—the correct procedure is to set it up so that the bubble is at the reversing point. This is the point at which the bubble remains as the instrument is rotated around on its vertical axis for an entire 360 degrees. As with many other alignments that are required on a surveying instrument, the system for making sure the mechanical vertical axis of the instrument is truly vertical can be used to level an instrument even if the bubble itself (the tubular vial) is out of adjustment. In an instrument where the vial is in adjustment, the axis of the level tube is exactly perpendicular to the mechanical vertical axis. Using the principle of reversal, this adjustment can be easily checked; in fact, this should be checked every time the instrument is leveled. Begin by simply following the normal procedure for leveling. Depending on the instrument and your preferences, rotate one or two leveling screws to center the bubble in its tube. Then rotate the instrument (or alidade) 180 degrees and observe the bubble’s position. The apparent movement, if any, shows twice the error in adjustment. Use the same leveling screw(s) you used to center the bubble to now move it back halfway to center. Now repeat this process beginning 90 degrees from your starting position. When you are finished, the instrument’s bubble will stay at the same position in the tube, no matter how the instrument is positioned. The bubble is now at the reversing point and the instrument is leveled.