Dioptra

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A dioptra is an instrument dating back to ancient Greece, at least 300 BCE. It is said to have been "long used by Greek astronomers", such as Hipparchus, sometimes credited with inventing it. Adapted to surveying, the dioptra is similar to the theodolite, or surveyor's transit, which dates to the sixteenth century. It is a more accurate version of the groma.

The dioptra may have been sophisticated enough, for example, to construct a tunnel through two opposite points in a mountain. It may have been a factor in building the Eupalinian aqueduct, "one of the greatest engineering achievements of ancient times," a tunnel 1,036 meters (4,000 feet) long, excavated through Mount Kastro on the Greek island of Samos, in the sixth century BC. Scholars disagree whether the dioptra was available that early.

An entire book about the construction and surveying usage of the dioptra is credited to Hero of Alexandria (also known as Heron; a brief description of the book is available online; see Lahanas link, below). Hero was "one of history’s most ingenious engineers and applied mathematicians."

The theodolite is superior to the dioptra because it has a compass and telescopic lens, two inventions unavailable to Hero.

The dioptra was used extensively on aqueduct building projects. Screw turns on several different parts of the instrument made it easy to calibrate for very precise measurements