The heliotrope is an instrument that uses a mirror to reflect sunlight over great distances to mark the positions of participants in a land survey. The heliotrope was invented in 1821 by the German mathematician Carl Friedrich Gauss.[1][2] The word "heliotrope" is taken from the Greek: helios (Greek: Ἥλιος), meaning "sun", and tropos (Greek: τρόπος), meaning "turn".
Heliotropes were used in surveys from Gauss's survey in GermanyColonel Sir George Everest introduced the use of heliotropes into the Great Trigonometric Survey in India around 1831,[3] and the United States Coast and Geodetic Survey used heliotropes to survey the United States. The Indian specification for heliotropes was updated in 1981,[4] and the American military specification for heliotropes (MIL-H-20194E) was retired on 8 December 1995.[5]
in 1821 through the late 1980s, when GPS measurements replaced the use of the heliotrope in long distance surveys.Surveyors used the heliotrope as a specialized form of survey target; it was employed during large triangulation surveys where, because of the great distance between stations (usually twenty miles or more), a regular target would be indistinct or invisible. Heliotropes were often used as survey targets at ranges of over 100 miles. In California, in 1878, a heliotrope on Mount Saint Helena was surveyed by B. A. Colonna of the United States Coast and Geodetic Survey from Mount Shasta, a distance of 192 miles (309 km).[6]
The heliotrope was limited to use on sunny days and was further limited (in regions of high temperatures) to mornings and afternoons when atmospheric aberration least affected the instrument-man's line of sight.[7] The heliotrope operator was called a "heliotroper" or "flasher" and would sometimes employ a second mirror for communicating with the instrument station through heliography, a signalling system using impulsed reflecting surfaces. The inventor of the heliograph, a similar instrument specialized for signaling, was inspired by observing the use of heliotropes in the survey of India.