Roman architects were skilled in this kind of leveling work, for which they used sophisticated tools. Besides the ordinary level, similar to the one used today by carpenters, they used devices such as groma, chorobates and dioptra.
Given the elementary means, materials en tools which were available, it is remarkeble to see the precision withwhich the Roman aqueducts were laid out. The mean gradient of a Roman aqueduct was something between 0,15 - 0,30 %.
Additional resources
For these and other measures, see a separate webpage.Literature:
- I. Moreno (2004): Roman Surveying (from the Spanish Traianus website)
- M.J.T. Lewis (2001): Surveying instruments of Greece and Rome
- K. Grewe (1985): Planung und Trassierung Römischer Wasserleitungen
- O.A.W. Dilke (1971): Roman Land Surveyors
- Roman military surveying, from YouTube
Rome, we have a problem
Occasionally the construction of a tunnel went awry, as recorded on a monument set up in 152 CE. by the ancient engineer of one such project, the aqueduct for the North African town of Saldae (today's Bougie, in Algeria). In Latin surprisingly circumstantial for being chiseled on stone, the engineer, Nonius Datus, recounts how, after he designed the project and left the site for other parts of the empire, two crews began digging at the opposite sides of a mountain. However, they swerved off the course he set for them, a problem noted when the crews had not met up after the combined distance of their tunnels exceeded the width of the mountain. The emperor summoned Nonius Datus back to the town to correct the problem. On his journey back to Saldae, he was robbed, badly beaten, and stripped of his clothes by highway brigands, but he eventually arrived and was able to direct the two crews to a successful rendezvous in the heart of the mountain. (For the inscription, see CIL 8.2728.) (from: Aicher 1995)
Groma
The principal Roman surveying instrument was the groma. It was regarded as the tool most typical of a surveyor; it appeared in stylised form on the tomb of Lucius Aebutius Faustus. L. Aebutius Faustus lived in the colony of Eporedia in northern Italy. He was a freedman (Hauck, 1988:42).The groma was used in military and civilian surveying, and we are told that a central point in a military camp was called the gromae locus (Dilke, 1971:66). Since no groma has survived completely intact, we do not have an accurate picture of one.
The one that appears on Lucius Aebutius Faustus' tomb serves as a starting point (see illustration of the tombstone of Lucius Aebutius Faustus). The staff of the surveying instrument is upright and the cross is detached and laid diagonally across it. There is not enough evidence to say for certain that this instrument is a groma, but the consensus is that it most likely is (Dilke, 1971:66). It certainly matches the description.
Chorobates
The chorobates was a bench with weighted strings on its sides for measuring the ground's angle on a system of notches, and a short channel in the centre, likely for testing the direction of the water flow (O'Conner, 1993: 45). It was mostly used for the levelling of aqueducts. It was probably too unwieldy for general levelling (Dilke 1971:76).It was also probably too unwieldy to use in the construction of tunnels, being too big to manoeuvre easily in confined spaces. See the illustration of a chorobates.
Dioptra
The dioptra was a different kind of level. It rested on the ground, and was finely adjusted by tilting and rotating the top part by means of precision screws, it could assess the angle of a stretch of aqueduct by looking through pivoting sights (O'Conner 1993: 45). See the illustration of a dioptra. Whether or not it was actually used is debatable, as only Hero of Alexandria - he lived during Nero's reign - gives us a description of the device. Vitruvius recommends the dioptra as an alternative for levelling water-courses and Pliny the Elder recognised its efficiency for astronomical work. Vitruvius' reservations and the lack of further written evidence suggests that it may have been regarded as too elaborate, expensive and unwieldy for general use (Dilke, 1971:79). As Hauck (1988:44) points out, the dioptra was essentially a forerunner of the modern theodolite. Despite its apparent complexity, it would have been useful in tunnels where the chorobates could not be used.
Other Surveying Instruments
The chorobates (see above) was already equipped with a plumb line.Another levelling instrument used by the Romans was the simple libella. It consisted of a frame in the shape of the letter A, with the addition of a horizontal bar on top (see photo). From the apex a plumbline was suspended that coincided with a mark on the lower crossbar when the instrument was level. Other marks could have been added to indicate other slopes, but there is no evidence that this was done (Hauck, 1988:43).
A portable sundial was found in Verus' workshop in Pompeii. Not only was this intended to indicate time, but lines on two of the sides were used for measurements. The exact use of the sundial is uncertain (Dilke, 1971:72). A sundial can be used for more than tracking time, it can also be used to orientate buildings.
Texts from Evan J. Demsky 2009
| HOME | More literature on more aqueducts | Last modified: March, 2016 - (webmaster) |