### 2. Major components

Although individual TSI designs vary, all have similar major components. These are shown in Figure A-1 and described below.

 Figure A-1 Major Components and Controls

The vertical and horizontal circles are used to measure vertical and horizontal angles respectively. Each has a lock and slow motion to allow accurate sighting.

The optical plummet is the modern equivalent of a plumb bob and is used to set up over a ground point. Is usually built into the TSI's base so it rotates with the instrument.

The circular (aka bull's eye) bubble, used for rough leveling, is fixed; the tube bubble, used for precise leveling, rotates with the instrument.

The tribrach is the lower assembly which by which the TSI is attached to the tripod and leveled. It consists of three leveling screws and (usually) a circular bubble. The instrument can be detached from the tribrach, Figure A-2, allowing a surveyor to swap pieces of equipment without having to re-level and re-center.

 Figure A-2 Tribrach Use

3. TSI position

Measurements are taken with the TSI in either the direct or reverse position. The difference between the two positions is the rotation of the telescope about the VA. By revolving the telescope about the VA and re-sighting the same target, the geometry of the TSI is reversed. Which is the direct and which is the reverse TSI position?

On TSIs with a display on a single side of the instrument, it is convenient to call the direct position the one in which the display faces the operator; the reverse position would be with the display opposite the operator.
On dual display TSIs generally the direct position is when the vertical circle is to the operator's left side. This is also referred to as circle left or face left. The reverse position would have the vertical circle to the operator's right (circle right or face right), Figure A-3.

 (a) Direct (b) Reverse Figure A-3 TSI Positions

Ultimately it really doesn't matter which position is direct or reverse. What's important, as we'll see in the following sections, is that by measuring in direct and reversed positions many instrumental errors are compensated.