The Romans are historically known as great builders. They constructed a massive network of roads, as well as huge public works, including aqueducts and temples. Their primary tools were designed to measure vertical and horizontal angles, distances and level. Many techniques and tools used by Roman engineers and surveyors were originally invented by prior civilisations, such as the Greeks and Egyptians, but evolved in terms of both craftsmanship and accuracy under Roman direction. Much of what is known about these tools today comes from the hard work of scholars pouring over ancient manuscripts.
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A simple but brilliant tool, these large rulers were employed in measuring land. Each 10-foot decempeda was made of a hardwood such as oak, which resisted warping, and capped on either end with bronze, allowing two decempedas to be fixed together to create a single larger ruler. A Roman surveyor would then "leapfrog" his decempedas, detaching the first and reattaching it to the butt of the second as many times as necessary to reach the desired measurement.
This surveyor's tool was used primarily to lay out straight lines and right angles over large distances, such as for roads. The groma consisted of a vertical upright, a perpendicular swivelling piece with a plumb-bob called a rostro, and a crosspiece of wood with hanging plumb-bobs called a groma. The upright would be placed in the ground and the surveyor would sight along the groma while making adjustments with the rostro, aligning the plumb bobs hanging from the arm with the one in the centre and describing a straight line. The process could then be repeated in other directions, providing the surveyor with a perfect ninety-degree angle. This process was used to divide land into plots, and to create the standard layout of Roman settlements, which centred on an intersection of two perpendicular roads.
The Roman dioptra was a workhorse for the surveyor or engineer. It was used as a level, a distance-measuring device and an angle-measuring device. Inherited from the Greeks, the dioptra was a complex instrument most likely employed only when very accurate measurements had to be taken over long distances, such as for the construction of aqueducts. It was also used for making celestial observations, and may have been an ancestor of the astrolabe and sextant. Consisting of a circular table covered with calibrated angles and attached to a tripod, once level, this device could be used to accurately measure the angles of terrain, even in the hardest-to-reach places.
The chorobates, commonly called the spirit level, was an easy-to-build but cumbersome levelling device. It was made from wood and consisted of a long bench with plumb-bobs hanging on each corner and a trough cut out of the top for water. The chorobates was levelled using the plumb-bobs in a manner similar to the groma. But because it was often built quickly and imperfectly, Vitruvius, its creator, added a failsafe: water was poured into a trough to help align the bench perfectly. Once the device was level, a sighting rod could be used to transfer that reading to the construction project that needed measuring. Unlike the dioptra, it was inaccurate at very long distances.
Other Roman Instruments
The Romans used other tools to help in the building of roads and aqueducts. One was a hodometer, which was a cart that was used to mark distance. The wheel was attached to a set of gears, and as the surveyor pushed the cart, the gears would tick off distance by pushing a small tooth on a measuring gear. After 399 small teeth, the gears would hit a large tooth, which would knock a pebble into a bowl in the cart and would mark that a Roman mile had been travelled. This repeated allowed surveyors to measure distances along roads to lay road markers. Another tool was a libra, which engineers think consisted of a set of scales and a sighting tube. By hanging the scales, level could be found, and the sighting tube would allow the surveyors to measure minute gradients needed to build aqueducts. This tool is not yet well understood by historians.
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