Roman Surveying

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Article taken from "Backsights" Magazine published by Surveyors Historical Society

Two learned Romans, Marcus Vitruvius Pollio, and Sextus Julius Frontinus, wrote of surveying practices in the Roman Empire at the time of Christ.  Undoubtedly there were more works from their time, but many classical works were irretrievably lost in the destruction of the Alexandrian library in 642 A.D.

chorobate.JPG (40624 bytes)Marcus Vitruvius Pollio, a master of architecture, presented De Architectura Libri Decem (10 books) to this patron Augustus Caesar, about 20 B.C.  Vitruvius wrote of the CHOROBATES, an instrument used for leveling hydraulic gradients to cities and houses.  The water supply for Rome alone was comprised of ten great a aqueducts, some coming from lakes as far as sixty miles from the city.  The CHOROBATES is described as a rod 20 feet long with duplicate legs attached perpendicularly at each end.  Diagonal pieces connect the rod and the legs, and both diagonal members have vertical lines scriven into them, over which plumb bobs are hung.  When the instrument is in position, and the plumb-lines strike both the scribe-lines alike, they show the instrument is level.  If the wind interferes with the plumb lines, the water level at the top of the horizontal piece is used.  Vitruvius instructs that the water level groove was to be "five feel 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.

Also in his writings, Vetruvius describes a device handed down from the "ancients" for measuring traveled distances by a counter fixed to the wheels of a chariot, similar to our odometer.

groma.JPG (67763 bytes)Sextus Julius Frontinus (c35-104 A.D.), a distinguished hydraulic engineer, authored De Aqui Urbis Romae Libri II.  It conveys in a clear and terse style much valuable information on the manner in which ancient Rome was supplied with water, and other engineering feats.  He also made the distinctions clear between the practices of the Roman "agrimensores" (field measurers) and "gromatici" (GROMA users).  The latter are named for the favored aligning instrument of the Romans (handed down from the Egyptians through the Greeks), resembling a surveyor's cross, that satisfied the bulk of their requirements - laying out straight lines and right angles.  The GROMA consisted of a vertical iron staff (ferramentum) about 5 feet long, pointed at the lower end, and with a cross arm, 10 inches long, pivoted at the top, which supported the main aligning element - the revolving "stelleta" (star) with arms about 3-1/2 feet across:  The two main roads at right angles in a Roman encampment were located by sighting beside the two plumb lines suspended from the end of the cross arms to coincide with the central plumb line over the selected central point.  Areas of fields were measured by settling out two right-angled lines, joining their extremities by straight lines and finding the perpendicular offsets from these to the irregular sides.  The metal parts of the GROMA, as well as rods and other equipment, were discovered in the ruined layers of Pompeii, in affirmation to Frontinus' descriptions.

An inspection of Roman roads, aqueducts, canals, buildings, city layouts, and land subdivisions confirms their unexcelled proficiency in the use of crude surveying instruments as measured by modern-day standards.  Further inspection of archeological and written evidence suggests the following points:

1.  The range of Roman instruments was restricted to the vision of the naked eye.  (Magnification by telescopic sights came in 1608).

2.  There is no evidence of the use of the compass.

3.  Large scale maps were greatly distorted in the E-W direction because the methods used for locating relative latitude and longitude were not sufficiently accurate for cartographical purposes.

4.  Their entire astronomical and geographical outlook was circumscribed by the idea of an earth-centered universe and a rigid Euclidean geometry excellent for earth measurements but elementary when projected into space.  They understood a great deal of algebra and trigonometry but very little calculus.


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