Chorbates (Leveling Instrument)

The 'Chorobates,' described by Vitruvius in Book VII of the Architecture, was used to measure horizontal planes and was especially important in the construction of aqueducts.

Similar to modern spirit levels, the 'Chorobates' consisted of a beam of wood 6 m in length held by 2 supporting legs and equipped with 2 plumb lines at each end. The legs were joined to the beam by two diagonal rods with carved notches. If the notches corresponding to the plumb lines matched on both sides, it showed that the beam was level. On top of the beam, a groove or channel was carved. If the condition was too windy for the plumb bobs to work effectively, the surveyor could pour water into the groove and measure the plane by checking the water level.

Vitruvius instructs that the water level groove was to be "five feet long, one digit wide, and a digit and a half deep". By using two or more 'Chorobates,' established levelly, the vertical distance between instruments could be established by sighting along the depth of the uphill instrument, to a rod placed atop the lower 'Chorobate.'

Libella (Leveling Instrument)

Roman architects were skilled in this kind of leveling work, for which they used sophisticated tools. Besides the ordinary level (Libella), similar to the one used today by carpenters, they used devices such as a groma, chorobates, and dioptra.

Given the elementary means, materials en tools which were available, it is remarkable to see the precision with which the Roman aqueducts were laid out. The mean gradient of a Roman aqueduct was something between 0,15 - 0,30 %.

Additional resources

Another leveling 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 plumb line 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).

Literature:

> I. Moreno (2004): Roman Surveying (from the Spanish Traianus website)

> M.J.T. Lewis (2001): Surveying Instruments of Greece and Rome

> O.A.W. Dilke (1971): Roman Land Surveyors

Roman military surveying, from YouTube

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