History Of Theodolite

Prior to the theodolite, instruments such as the geometric square and various graduated circles and semi-circles 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 described such an instrument in his book, Margarita Philosophica, which he published in Strasburg in 1512.^[1]

The first occurrence of the name theodolite, or 'theodelitus', is found in the surveying textbook A geometric practice named Pantometria (1571) by Leonard Digges. This was published posthumously by his son, Thomas Digges. Digges senior invented the name, but its origin is unclear.^[1]

There is some confusion about the instrument to which the name originally applied. Some identify the early theodolite as an azimuth instrument only, while others specify it as an altazimuth instrument. In Digges' book, the name theodolite described an instrument for measuring horizontal angles only. He also described an instrument that measured both altitude and azimuth, which he called a topographicall instrument [sic].^[2] Thus the name originally applies only to the azimuth instrument and only later became associated with the altazimuth instrument. Even as late as the 19^th century, the instrument for measuring horizontal angles only was called a simple theodolite and the altazimuth instrument, the plain theodolite.^[3]

The earliest altazimuth instruments consisted of a base graduated with a full circle at the limb and a vertical angle measuring device, most often a semi-circle. An alidade on the base was used to sight an object for horizontal angle measurement and a second alidade was mounted on the vertical semi-circle. Later instruments had a single alidade on the vertical semi-circle and the entire semi-circle was mounted so as to be used to indicate horizontal angles directly. Eventually, the simple, open-sight alidade was replaced with a sighting telescope. This was first done by Jonathan Sisson in 1725.^[3]

The instrument of theodolite became a modern, accurate instrument with the introduction of Jesse Ramsden's famous great theodolite, completed in 1787.^[3] 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. This, with continuing refinements, evolved into the modern theodolite used by surveyors toda