Heliotrope, Transit, and Chain
Several tools were key to a surveyor’s work. The surveyor’s chain was an early measurement tool. The chain measures 66 feet and is made of iron to keep it functional despite being dragged across rough terrain. The rings between each link were made in such a way that the chain could never tangle when thrown.
The heliotrope (left) was invented by the famed mathematician Friedrich Gauss in 1821 to measure longer distances. This instrument uses a small mirror to reflect sunlight to another surveyor or station, and, through triangulation, measures the distance between the two points. Thus, the name “heliotrope” is derived from the Greek words “helios” meaning “the sun” and “tropos” meaning “to turn.” Consequently, the heliotrope could only be used on sunny days, but its usefulness outweighed this drawback.
A surveyor used a transit (right) to determine angles and measure straight distances. The transit comprised a telescope, a vernier scale (similar to what we would call a protractor), and a compass, and was used in conjunction with the heliotrope. A surveyor on the other side of the heliotrope would line up the transit with the heliotrope using the compass to make sure a straight line was made. Then the telescope was used to sight the mirror reflecting the light. The vernier scale is attached to the telescope so that moving the telescope to find the mirror would give the surveyor the angle between the ground and the light down to the minute of an angle. Using these angles, surveyors could determine the distance between the two points.