Roman surveying instruments were generally adapted from devices used by the ancient Greeks. By borrowing and refining these Greek surveying methods, the Romans were able to carry out incredibly complex and precise construction projects. The Roman agrimensores, or "land measurers," used these surveying tools to plan and build a variety of structures, such as temples, roads and aqueducts.
The groma, sometimes known as the surveyor's cross, was the most widely-used Roman surveying tool. The groma consisted of a wooden cross attached to a staff. The staff was placed vertically into the ground, after which the surveyor would use the arms of the cross to calculate straight lines and right angles. The groma had many uses, but was particularly important for planning Roman roads.
The dioptra, like the groma, was an angle measuring tool. It was more sophisticated than the groma, and could be used to calculate angles and gradients as well as distances. By rotating a sighted bar around a central table, the surveyor could calculate angles and distances in regards to a secondary object. A number of small water levels attached to the instrument helped the surveyor to keep the dioptra perfectly level.
The chorobates was a basic Roman spirit level. It had a variety of uses, but was particularly important when surveyors needed to create large tracts of level ground in advance of further construction projects. The instrument consisted of a series of plumb bobs hung from a rod of about 20 feet in length. Levels were then measured by aligning the plumb bobs with vertical lines on the chorobates.
The decempeda was a simple wooden rod used for measuring distances. Each graduated rod measured 10 Roman feet in total. Two rods were used for measuring greater lengths, with each rod being laid down in turn, one after the other.
For measuring greater distances, particularly those along planned-out or existing roads, Roman surveyors used an instrument called a hodometer. This wheeled device was attached to the side of a cart. When the cart was pushed along, a gear attached to the hodometer's wheel dropped a stone into a bowl after the completion of every Roman mile. This device was commonly used to calculate the distance between cities, with a permanent milestone placed after every mile.