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