1. Introduction

A Total Station Instrument (TSI) consists of an EDM integrated with a digital theodolite combining distance and angle measurement into a single package. EDM principles (and TSI integration) are more fully discussed in the Distance Measurements topic.

A TSI is one of the most important and versatile pieces of equipment in a surveyor's armory. It can be used to collect or stake out data in all three dimension under varying conditions. As the technology evolves, greater computing and storage capacity are being built into TSIs increasing their flexibility and usefulness. Recent innovations include reflectorless distance measurement, greater robotic TSI sophistication, and even GPS integration.

This topic will deal with a basic TSI and concentrate on set up, simple field operations, error control, and best practices. Once comfortable with basic TSI set up and use, the surveyor should delve into the instrument's manual where advance capabilities and field operations are discussed. As with any surveying operation, repeated practice increases efficiency and decreases errors.

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, 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.


 img050        img051
(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.