The most significant errors affecting the operation of a theodolite are geometrically cancelled by recording angle readings in both direct and inverted positions. However, that does not negate the benefits of a well-calibrated instrument. In practice, many observations are made on one side only (in topographic mapping, for example). Also, with a finely-tuned instrument, a discrepancy in the readings will help identify other problems (e.g., an unstable tripod, observation or recording blunders). The mechanics of the calibration procedure will depend on the theodolite. This is only a discussion of the underlying geometry. Be aware that an electronic theodolite may have a routine that will measure these errors and adjust the readings accordingly. This has certain advantages over mechanical adjustments. No weather seals are broken, there is no wear on the screws, and there is little chance of inadvertently introducing some other error in the adjustment process. Four calibration procedures are given below. Their order is important, particularly for the last two on the list. If a steeple check were performed before the double deflection, then two errors would be observed together, and it would not be possible to isolate them. 1. 2. 3. 4.
Vertical Axis — Leveling the Instrument Vertical Collimation Line of Sight Perpendicular to Trunnion — Double Deflection Check Trunnion Axis Level — Steeple Check
1. Vertical Axis — Leveling the Instrument The first step in calibrating an instrument is to see that it is properly leveled. If an instrument is not leveled, then the vertical axis is not plumb, and that is the axis about which horizontal angles are measured. The instrument or (more likely) the Tribrach may have a circular bull’s-eye leveling vial, which is precise enough for centering and for low-order work. The tube leveling vial is much more precise. The vertical axis must be perpendicular to this vial. Imagine a line running...