Theodolite

Theodolite (pronounced thee-od-l-ahyt)

(1) In surveying, a precision instrument having a telescopic sight for establishing horizontal and sometimes vertical angles; usually called a transit in US & Canada.

(2) As phototheodolite, an instrument consisting of a theodolite mounted on a camera which can take at each of several stations of known position and elevation (as determined by transit survey) a series of photographs used in terrestrial photogrammetry.

1571: From the New Latin theodolitus the origin of which is contested.  It was probably coined either from the Arabic al-idhâda (a sort of rule) or the Ancient Greek theā (a viewing) plus a second element of unknown origin which may have been invented.  The North American use of” transit” rather than theodolite is historic.  During the nineteenth century, Americans preferred the surveyor’s compass and, later, the surveyor’s transit, which were cheaper and more robust and, despite technical progress, the name stuck.

Many have attempted to trace the etymology of theodolite but no satisfactory explanation has been found.  The first use was in a book by English mathematician and surveyor Leonard Digges called Geometrical practical treatize, named Pantometria, diuided into three bookes, longimetria, planimetria, and stereometria (1571, second edition 1591).  The author didn’t mention any derivation and used the spellings “theodelitus” and “theodolitus” alternately.  Writing a few years later, another mathematician named the same instrument a “horizontall or flatte sphere”, although inconsistencies in spelling was at the time not unusual, the same author when speaking of an alidade (a sighting device or pointer for determining directions or measuring angles used in both surveying and astronomy), spelled it as “alideday” and “athelida”.

One explanation is that both theodelitus and athelidae were corruptions of the Arabic al-idhâda (a sort of rule), and both mathematicians would have been acquainted with the work of earlier Arabic mathematicians.  That’s not unconvincing for athelidae but theodelitus is more likely to have been derived from (or at least influenced by) the Ancient Greek; the second element has mystified all.  The first part of the New Latin theo-delitus might stem from the Ancient Greek θεᾶσθαι (to behold or look attentively upon) or θεῖν (to run) but the second part is puzzling though sometimes attributed to an unscholarly variation of one of the following Greek words: δῆλος (evident; clear), δολιχός (long) or δοῦλος (slave) or even an unattested Neo-Latin compound combining ὁδός (way) and λιτός (plain).  It has been also suggested that -delitus is a variation of the Latin supine deletus, in the sense of "crossed out".

From mechanical to digital

Copper & brass theodolite, Hall Brothers of London, circa 1894.

A theodolite is an optical device used to measure angles between visible points in the horizontal and vertical planes.  Historically they were used in land surveying but they’re used also in all aspects of civil engineering, meteorology and rocket telemetry.  The earliest theodolites were small mounted telescope which rotated horizontally and vertically; modern versions are sophisticated digital instruments which can track weather balloons, airplanes, and other moving objects, at distances up to 12 miles (20 km / 65,600 feet).  Nearly always used mounted on a tripod and originally mechanical-optical instruments, almost all theodolites in active use are now digital.  Theodolites, whether mechanical or digital, are instruments used to determine the relative position of points on the earth's surface by measuring the horizontal and vertical angles. Digital theodolites are more convenient and accurate as they provide the operator with readouts.  With the traditional device, it was necessary manually to read the numbers directly from a graduated circle.

The modern digital theodolite.

A digital theodolite consists of a telescope mounted on a base, a sight at the top used to align the target, vision of which is clarified by adjusting a focusing knob, an objective lens on the opposite side used to sight and magnify the target as required.  Despite the differences in construction, the parts of a digital theodolite are mostly similar to the non-digital theodolite, except for the addition of a liquid crystal display (LCD) and the operating keys for changing the settings.  In the same manner as the non-digital theodolite, leveling is performed using optical plummets or plumb bobs and the spirit or bubble level.  In use, it’s made first to stand vertically above the survey point with the help of a plumb bob or optical plummet and is then made in level to the horizon with the help of internal spirit levels.  Once the leveling process is complete, the telescope is used to focus on the target and the respective horizontal and vertical angles are displayed on the screen.