The dioptra was first used by the Greek astronomers to fix the position of the stars. However, when this was superseded by a more accurate device, the armillary sphere, it found a new function as a surveying tool. It was a portable instrument that could be used to measure distances, angular measurements or work as a level. The dioptra could assess both the horizontal and vertical planes, allowing for readings of the deviation between two set locations, so offering accurate measurement of any angles involved.
