An instrument designed for measuring horizontal and vertical angles, the theodolite is of critical importance to the surveying profession. Comprising a telescopic lens that can tilt on both horizontal and vertical axes, the theodolite is essential in "triangulation," a long- established technique for surveying tracts of land. The distance between two points is measured, to become the baseline of a triangle. The theodolite is placed at one end of the line and used to determine the angle to a predefined distant point (which makes the third corner of the triangle). The instrument is then moved to other end of the baseline, where a second angle is measured to that same point. Simple trigonometry is then employed to calculate accurately the length of the other two sides.
The word theodolite first appears in a surveying textbook, A Geometric Practice Named Pantometria (1571) by mathematician and scientist Leonard Digges. Although tools for measuring angles had existed for centuries, it was Digges who introduced an "altazimuth" instrument—one that could be moved on two perpendicular axes. The inventor himself died penniless twelve years before the publication of his book. His historical standing can be attributed to his son, Thomas, who worked tirelessly to continue his father's work, and who would become a significant figure in his own right, popularizing the principles of modern science.
There have since been many technological refinements, namely Jesse Ramsden's Great Theodolite of 1797, used in the Principal Triangulation of Great Britain—the original Ordnance Survey.
