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.
A: Today’s automatic level compensators are fairly rugged. Some depend on other systems than fine wires to support the movable prism(s) or mirrors. Ball bearings, magnetic levitation and plastic tapes are some of the alternate technologies. As with all surveying instruments, automatic levels must be protected from shock, vibrations and extreme and rapid changes in temperature, humidity, etc.
The two-peg test functions as an excellent basic way of determining the ability of the instrument to give good leveling results. This simple test can be performed by setting up halfway between two points (about 100 m apart) and checking the elevation difference between them. Then set up about 2 m away from one of the points and check the elevation difference again. If the level is in adjustment, the elevation difference will be the same for both setups. This applies for all types of levels. If you wish to check the compensator function, sight at a level rod about a 100 feet away. It doesn’t need to be vertical, so it can be leaned against a building. Make sure that the telescope passes directly over one of the leveling screws. Note the rod reading, then slowly and gently rotate the leveling screw that is directly under the line of sight. This rotation will cause the mechanical axis of the telescope to move up or down, away from the horizontal. If the compensator is working properly, you will see a slight movement of the crosshairs up or down, and then a movement as the compensator works to bring the crosshair back to the original rod reading. If the crosshairs move up or down when you turn the leveling screw, and do not return to the original rod reading, you have a faulty compensator. Continue turning the leveling screw until you see the action of the compensator cease. This indicates the compensator’s working range. Note the position of the bubble in your circular vial; this gives you an indication of the range of the compensator, if you know the sensitivity of your circular vial. If by this test your compensator appears to be faulty, a qualified service shop will be required to service it.
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