The permanent adjustments of a theodolite are procedures used to calibrate the instrument and ensure its accuracy over an extended period of time. The following are the main permanent adjustments of a theodolite:
Vertical Axis Collimation: This adjustment ensures that the line of sight of the instrument is aligned with the center of the vertical axis of the instrument. This is a critical adjustment as it affects the accuracy of all vertical angle measurements taken with the instrument.
Horizontal Axis Collimation: This adjustment ensures that the line of sight of the instrument is aligned with the center of the horizontal axis of the instrument. This is a critical adjustment as it affects the accuracy of all horizontal angle measurements taken with the instrument.
Centering: This adjustment ensures that the axis of the vertical circle is centered over the axis of rotation of the theodolite. This is a critical adjustment as it affects the accuracy of all vertical angle measurements taken with the instrument.
Index Error Correction: This adjustment ensures that the vertical axis of the theodolite rotates exactly 90 degrees when the instrument is turned from one vertical setting to the other. This is a critical adjustment as it affects the accuracy of all vertical angle measurements taken with the instrument.
Tangent Screw Error Correction: This adjustment compensates for any error in the movement of the tangent screw, which is used to control the vertical movement of the theodolite. This is a critical adjustment as it affects the accuracy of all vertical angle measurements taken with the instrument.
These permanent adjustments are performed by trained and certified technicians and are critical to ensuring the accuracy and reliability of the instrument over an extended period of time. They are usually performed at regular intervals, such as annually or after the instrument has been dropped or subjected to other extreme conditions.
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