Heliotrope (instrument)

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 by the German mathematician Carl Friedrich Gauss. The word "heliotrope" is taken from the Greek: helios, meaning "sun", and tropos , meaning "turn". It is a fitting name for an instrument which can be turned to reflect the sun toward a given point.

The heliotrope was utilized by surveyors as a specialized form of 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 appear indistinct. Heliotropes have been used repeatedly 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 USCGS from Mount Shasta, a distance of 192 miles (309km).



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 did not affect the instrument-man's line of sight. 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.