1. Instrument axes

a. Identification

Refer to Figure B-1 for visual depiction of the following axes.

Vertical axis (VA). Axis about which the TSI rotates in a horizontal plane. The horizontal circle is perpendicular to the vertical axis.

Horizontal axis (HA). Axis about which the telescope rotates in a vertical plane.

Line of Sight (LoS). The line along which a sight is made. It is defined by a line connecting the crosshair intersection and the center of the telescope's objective lens, just as for levels.


Figure B-1
TSI Axes


Optical plummet axis (OPA). This is also a LoS except it is bent. Like the LoS, the OPA is defined by the plummet's sighting mark and optics. Along its path it intersects a prism which deflects it 90°, Figure B-2. 

Figure B-2
Optical Plummet Axes


Axis of the bubble tube (ABT). A line tangent to the center of the upper curved surface of the tube bubble, Figure B-3.

Figure B-3
Axis of the Bubble Tube

b. Relationships

On a perfectly adjusted TSI:

  • HA and ABT are parallel and both perpendicular to the VA, and,
  • LoS is perpendicular to the HA, and,
  • OPA coincides with the VA

When the TSI is level, then theoretically:

  • HA, ABT, and initial part of the OPA are horizontal, and,
  • VA and deflected OPA are vertical, and,
  • LoS rotates in a vertical plane.

In reality, because these axes represent mechanical aspects of the TSI, the relationships and orientations may be off. Any maladjustment represents a systematic measurement error. As we describe TSI use, we will address these errors, their effect on angle measurements, and how they can be overcome.