Groma

The groma was a surveying instrument used across the Roman Empire to lay out straight lines and right angles for dividing land into rectangular grids, a practice known as centuriation. It consisted of a rotating horizontal cross fitted with plumb bobs hanging from each arm, which a surveyor sighted along to establish accurate right angles. The groma is the only Roman surveying tool of which physical examples survive, and though similar land division existed earlier, Rome carried out centuriation on an unprecedented scale beginning in the second century BC to distribute land to veterans and citizens.