## 2. Vertical Axis

The vertical axis, VA, of an instrument is the line about which the instrument is rotated horizontally. Figure E-5 shows a total station instrument and automatic level with their respective Vertical Axes.

 a. Total Station b. Automatic Level Figure E-5 Instrument Vertical Axes

On a correctly adjusted bubble-based instrument the ABT is perpendicular to the VA. The instrument is oriented to gravity by centering the bubble which makes the ABT horizontal and the VA vertical, Figure E-6.

 Figure E-6 Correctly Oriented Instrument

If the ABT is not perpendicular to the VA then set up error is introduced. In later topics on specific instruments, we will see how to check and compensate for this maladjustment. For now, we will assume the respective instrument is in adjustment.