The Complete Handbook of Theodolites

Chapter 1: Understanding Theodolites

Theodolites are precision instruments used to measure horizontal and vertical angles. The name comes from the fact that they are "the" instrument for "dolite" (a word loosely derived from the Greek for "clear sight"), but that etymology matters far less than the function. At its core, a theodolite is a telescope mounted on a rotating base, equipped with graduated circles that allow a user to measure the angle the telescope is pointing at with a high degree of accuracy. That description sounds almost simple, but the history of surveying and construction is largely the history of making that measurement more accurate, faster, and more reliable.

You use a theodolite when you need to know, with certainty, the exact angle between two points relative to a known reference direction. A framing square tells you if a corner is roughly 90 degrees. A protractor on a drawing tells you the angle on paper. A theodolite tells you that a point on a hillside 400 meters away is sitting exactly 27 degrees and 15 minutes to the east of a fixed reference point, which is a measurement you can stake out, build against, and defend if a dispute arises. This is not a measurement you can derive from GPS alone. A standard GPS receiver provides a position, but it does not tell you the precise angle of a building's corner to the nearest arc-second. For that, you need an angle measuring instrument.

To understand why theodolites matter, you have