What Is a Theodolite?
A theodolite is a surveying instrument used for measuring horizontal and vertical angles, and has been adapted for specialized purposes such as meteorology and rocket launch. To measure long distances we use a system known as triangulation – we can use it, for instance, if we want to know the distance to the moon.
Astronomers measure the angle of the moon above the horizon at two places on the earth’s surface – as far apart as possible and at the same moment. This gives them a triangle and a base line (the distance between the two observations).
Since they have measured two angles of the triangle, they know the third, because the three angles of a triangle always add up to 180 degree. They therefore have enough information to find the distance between the apex (moon) and base line (earth).
The same system is used in surveying and map-making, but the surveyor’s base line is, of course, much shorter, in fact, both ends of the base line must be within sight of one another and of the next point to be observed. The theodolite is the basic instrument used in surveying, especially in precise triangulation.
A modern theodolite consists of a movable telescope mounted within two perpendicular axes: the horizontal or trunnion axis and the zenith axis. A theodolite measures vertical angles as angles between the zenith, forwards or plunged—typically approximately 90 and 270 degrees. When the telescope is pointed at a target object, the angle of each of these axes can be measured with great precision, typically two seconds of arc.
A theodolite may be either transit or non-transit. In a transit theodolite, the telescope can be inverted in the vertical plane, whereas the rotation in the same plane is restricted to a semi-circle in a non-transit theodolite. Some types of transit theodolites do not allow the measurement of vertical angles.
The builder’s level is sometimes mistaken for a transit theodolite, but it measures neither horizontal nor vertical angles. It uses a spirit level to set a telescope level to define a line of sight along a horizontal plane.
Prior to the theodolite, instruments such as the geometric square and various graduated circles (see circumferentor) and semicircles (see graphometer) were used to obtain either vertical or horizontal angle measurements. It was only a matter of time before someone put two measuring devices into a single instrument that could measure both angles simultaneously.
Gregorius Reisch showed such an instrument in the appendix of his book Margarita Philosophica, which he published in Strasburg in 1512. It was described in the appendix by Martin Waldseemüller, a German topographer and cartographer, who made the device in the same year. Waldseemüller called his instrument the polimetrum.
The theodolite became a modern, accurate instrument in 1787, with the introduction of Jesse Ramsden’s famous great theodolite, which he created using a very accurate dividing engine of his own design. The demand could not be met by foreign theodolites owing to their inadequate precision; hence all instruments meeting high precision requirements were made in England. Despite the many German instrument builders at the turn of the century, there were no usable German theodolites available. A transition was brought about by Breithaupt and the symbiosis of Utzschneider, Reichenbach and Fraunhofer.
As technology progressed, in the 1840s, the vertical partial circle was replaced with a full circle, and both vertical and horizontal circles were finely graduated. This was the transit theodolite. Theodolites were later adapted to a wider variety of mountings and uses. In the 1870s, an interesting waterborne version of the theodolite (using a pendulum device to counteract wave movement) was invented by Edward Samuel Ritchie. It was used by the U.S. Navy to take the first precision surveys of American harbors on the Atlantic and Gulf coasts.
In the early part of the 20th century, Heinrich Wild produced theodolites that became popular with surveyors. His Wild T2, T3, and A1 instruments were made for many years, and he would go on to develop the DK1, DKM1, DM2, DKM2, and DKM3 for Kern Aarau company. With continuing refinements instruments steadily evolved into the modern theodolite used by surveyors today.