SURVEYING INSTRUMENTS AND MEASUREMENTS OF ANCIENT ROME
Community College of Southern Nevada
Building Technology - Land Surveying Emphasis Degree Program
Throughout modern history, the Roman Empire has been credited with being the greatest builders of ancient times. Even up until the eighteenth century, no other civilization made as many remarkable advances in engineering and construction as the Romans. Although many of the instruments and ideas the Romans used were invented by other ancient civilizations such as the Babylonians, Egyptians and Greeks, the Romans perfected this ancient engineering technology. The Romans became the forerunners in accuracy and precision in laying out land and equally impressive, in building roads and constructing aqueducts.
The road system of Ancient Rome was one of the great accomplishments of its era. Like many other innovative ideas of ancient time, the Greeks and Etruscans constructed many roads before the Romans, and again the Romans utilized the techniques used by these earlier civilizations. However, the size and construction of the Roman road system was far superior to other ancient cultures.
There were four types of Roman roads and they were classified based on their importance in the Empire. Viae publicae, or public roads, were the principal roadways of Rome. Viae militares, or military roads, were built for the use of the Roman Army. The actus was a secondary type of road and privatae, or private roads, were usually constructed and maintained by the owners of the land these roads occupied.1
Many of the roads in Rome were paved and graded, which made them more durable. Some of these roads still exist today. The length of the road system of Rome was also a major accomplishment of its time. About 300 A.D., the Roman Empire had over 370 partially paved major highways, totaling 53,000 miles. In addition, there were thousands of smaller roads branching out from the main roads. Altogether, these roadways totaled hundreds of thousands of miles.2
One of the extraordinary achievements that elevated the Romans to the stature of the greatest builders of ancient times was their success in constructing aqueducts to supply large quantities of water to major towns and cities. Following the Roman strategy of learning from their predecessors, they implemented the ideas and designs of earlier civilizations in their construction of aqueducts. The Babylonians first created aqueducts and the Greeks built them as well. However, just like the road system, the Romans excelled over their predecessors in their system of aqueducts and water distribution. There were many aqueducts throughout the Roman Empire, with the city of Rome having eleven alone. The eleven aqueducts transported water a total of more than 260 miles.3
To create the ancient engineering wonders of road systems and aqueducts, the Romans employed professionals known as Agrimensores. The Agrimensores were land surveyors of Ancient Rome. Agrimensores, meaning "measurers of land", performed their work with more careful planning and more accuracy than any country at any time until the late eighteenth century.4
In many ancient civilizations, parts of the body were used for equivalents for the measuring system. The Romans implemented this type of system also. Because of the vast size of the Roman Empire, the measurements may have varied slightly from region to region. The basic measurement was the Roman Foot, also called the Pes. For this research paper, 1 Roman Foot is equivalent to 29.59 centimeters or .971 U.S. Survey Feet.5 The digitus was a measure of one finger-breadth, which is equivalent to .061 U.S. Survey Feet. 16 finger-breadths is approximately 1 Roman Foot. The foremost measurement of length used by the Roman Agrimensores for plotting out land was 120 Roman Feet, also called the actus. There were two commonly used measurements of area in the Roman Empire. The Iugerum, approximately 5/8 acre, was measured as the amount that could be ploughed in one day. The Century, approximately 125 acres, was a square 20 x 20 actus. The following table illustrates the conversion factors of Roman measurements used for this project:
The most widely used surveying instrument in Roman times was an instrument called the Groma. This instrument was used to lay out straight lines and right angles. Since roman lands were laid out on a rectangular grid, the Groma was the instrument of choice. The Groma was originally created by the Etruscans or possibly by the Greeks, but it was the Romans who perfected it and the method for using it. The best preserved remains of the Groma were discovered in 1912 in Pompeii in the workshop of a Roman Agrimensore named Verus. As with many instruments of the Ancient world, the wood parts of the Groma have disintegrated and only the metal parts remained. Therefore, the dimensions of the Groma, and the design itself, are only an estimate of what scientists and archeologists believe the actual Gromas used by the Romans may have looked like.
There are four major parts to the Groma: the cross, the plummets, the staff and the swing arm. The cross was at the top of the Groma and each arm of the cross was of equal length from the center, approximately 15-18 inches. Cords from the end of each arm suspended the plummets. The plummets were usually Roman plumb bobs, similar to the ones used by surveyors today, however they may have also used other types of weights. These weights may have all been the same type or they may have been in sets of two. The arms of the cross were reinforced at the center and the ends by metal angle brackets to protect the wood. The cross of the Groma was mounted to the staff by means of a swing arm, which was placed on a bracket that fitted into a metal collar that would then be set on the top of a wood staff. The staff was approximately 6-7 feet in height. The bottom of the staff was a heavy pointed iron tip, which was tapered to aid in placing it firmly in the ground.
The Dioptra was another surveying instrument that was utilized by Roman Surveyors. It was used to measure horizontal and vertical angles and could also be used as a leveling instrument and for astronomical observations as well. As with most instruments used by the surveyors of the Roman Empire, the Dioptra was not an original invention of the Romans, but was developed by the Greeks and implemented by the Romans. There are very few remains of the Dioptra and the estimation of the replication of this instrument vary from reference to reference. The best recollection of the description of what the Dioptra may have looked like and its uses comes from the records of Hero of Alexandria. Hero, a Greek, described the use of the Dioptra, however many of the important details regarding the construction of the instrument were either lost or destroyed. Since there has not been any concrete evidence to the contrary, it is believed that the Romans used the Dioptra in much the same way as the Greeks did and the design of the Roman Dioptra was parallel to that of the Greek origin.
The staff of the Dioptra may have been a three-legged instrument, similar to today's tripod or possibly a monopole with three points at the base to assist in keeping it vertical. Either of these types of staffs were approximately the equivalent of 7-7 U.S. Survey Feet in height. At the top of the staff, was a circular plate with a cylinder attached to it. This plate and cylinder allowed for rotation. At the top of the cylinder was another circular plate, approximately 30 inches in diameter. This plate was made of wood and was divided and marked into 360 degrees. Although the Dioptra was divided into degrees, it is believed that these measurements were mainly used for astronomical observations. However, the Greeks and Romans did have the knowledge and ability to use trigonometry, so it is possible that they used the Dioptra for horizontal angle measurement as well.
Another instrument used for leveling that could be placed on the tripod or monopole instead of the Dioptra was the water level. This instrument was a linear bar made of wood and was approximately the equivalent of 6-7 U.S. Survey Feet in length, 1-1/2 inches thick and 2 inches in width. In the center of the instrument, a groove was cut. A vertical metal tube would be placed in the groove and the ends of the tube would protrude upwards approximately 1-1/2 inches from each end of the wooden bar. A flat bar placed over the metal tube would assist in keeping the tube firmly in place. In each end piece of the tubing, a glass cylinder would be fitted. This cylinder was approximately the same height as the metal tube. The glass cylindrical tube would be filled with water. At the base of each end piece, a revolving rod or screw would be placed. These screws would be used to move the instrument up or down until the surveyor determined it to be level.
An alternative method of leveling implemented by Roman Surveyors was by using an instrument call a Chorobate. Since this instrument was made primarily of wood, there are virtually no remains still in existence today. However, the descriptions for the design of the Chorobate and its use have been recovered. It is believed that the Chorobate was used mainly for leveling in aqueducts. Because of its massive size and weight, it would not have been as practical and appropriate as the Dioptra in laying out roadways.
The Chorobate was approximately 20 Roman Feet in length and was made predominantly of wood. The Chorobate was elevated from the ground by wooden supports approximately 1-2 Roman Feet in height. At the base of the supports were wooden braces, which had vertical marks across the face. Plumb lines were hung from the supports and when the plumb lines on each end of the Chorobate touched the respective vertical marks simultaneously, the instrument would be level. In windy conditions, a different method for using the Chorobate could be implemented. A groove approximately 5 Roman Feet in length, a digit wide and a digit and a half deep, would be carved into the center of the Chorobate at equal distances from each end of the instrument. Water would be poured into the groove. When the water touched the upper edge of the groove equivalently, the instrument would be level.
A third type of leveling instrument used by the Romans was called the Libella or Libra. This was a type of hand level that was used primarily by carpenters and masons, but may have also been employed by the surveyors. This instrument consisted of an "A" shaped wooden frame. A plumb line was attached to the apex of the instrument. When the two vertical legs of the Libra were placed on a horizontal plane, the plumb line would cover a vertical line drawn across the front of the crossbeam. The top surface of the crossbeam would be used to take readings.
Leveling rods were used in conjunction with both the Dioptra/water level and the Chorobate. The rods used by the Roman Surveyors may have been quite smaller to the ones used by modern surveyors, however, as with other instruments of this era, there are insufficient remains of the design of the leveling rods. Therefore, the design and dimensions of the leveling rods is only an estimate of what the actual rods used in Roman time may have been.
The leveling rods were approximately 12-14 U.S. Survey Feet in length, 2 inches thick, and 4 inches in width.6 In the middle of the rod, and extending from one end of the rod to the other, was a groove with a slide in it. Attached to this slide was a target, approximately 9 inches in diameter, made of wood. The target was divided into two equal parts. This was accomplished by drawing a line across the target at right angles to the rod. One semicircle was painted black and the other semicircle was painted white. A cord was then fastened to the slide. This allowed the target to move up and down the rod by means of a pulley that was mounted on the top of the rod. Beginning at the lower end of the rod, marks were placed in intervals of digitus, palmus and Pes. Attached to the target was a pointer and by raising or lowering the target, the surveyor using the Dioptra could signal to the surveyor holding the rod at which point along the rod he was level. To hold the rods perfectly vertical, plumb-bobs were utilized.
The measuring tools used by the Romans were simple in design, yet their accuracy was remarkable, even by today's standards. The standard method of measuring distances in the Roman Empire was by the use of the measuring rod. Cords or ropes were used for measuring longer distances and the Hodometer was used to measure Roman miles.
The measuring rod used by the Romans was a Pertica or Decempeda, which was 10 Roman Feet, equal to 9.871 U.S. Survey Feet. It was made of wood and had metal end pieces, which were tapered. The ends of the rod were flat and circular that enabled the rods to be aligned with each other to measure distances longer than 10 Roman Feet. For smaller measurements, the metal end pieces of the rod were marked off. The first end piece was marked off in inches and half inches of the Pes. The other end piece was marked off in inches of the Pes and in digits or finger-breadths (16 finger-breadths to 1 Roman Foot).7 A plumb-bob could be used with the measuring rod for convenient and accurate linear measurements over uneven terrain or sloping ground.
As stated earlier, there were hundreds of thousands of miles of roads throughout the Roman Empire. Using the measuring rod to lay these roads out would have been a very long and tedious process for the Romans. Therefore, ropes or cords were utilized. There is no record of how long these ropes and cords were, but they were estimated to be either 60 or 120 Roman Feet. In order to keep the ropes from shrinking or lengthening, they were stretched between two stakes or hung vertically with weights attached to it for a certain length of time. This process was repeated several times. After this procedure was complete, the rope would then be smeared with a wax substance. This same method was implemented for the cord also. The accuracy of these measuring devices was possibly equivalent to the Gunter's chain or the Engineer's chain.8
Another type of measuring device used by ancient engineers and surveyors was called a Hodometer. This device was probably developed by the Greeks, but was also used by the Romans. The Hodometer had two special wheels, each one four Roman Feet in diameter. Every 400 revolutions a small pebble would drop into a metal bowl, which would mark 1 Roman mile.9 Most of the references used for this project agreed that the Hodometer was much too complicated and bulky to be used in the daily operations employed by the Agrimensores. Due to the sheer size of the Hodometer and the circumstances for which it was used, it was not included as part of the research and design for the student competition.
The Agrimensores were instrumental professionals in the design and construction of the great road and aqueduct systems of Ancient Rome. The instruments these surveyors used in their daily work were simple in design, yet the result obtained through the use of the Groma, Dioptra, and other instruments of the time, has amazed mankind for centuries. Although earlier civilizations such as the Greeks, Babylonians, and Egyptians created the majority of the ideas, tools and instruments used by the Romans, it was the Romans who perfected this ancient engineering technology and the methods for using and implementing it. The Romans excelled over their predecessors in their system of aqueducts and water distribution. The size and construction of the Roman road system was far superior to any other ancient culture. Because of these extraordinary achievements, the Romans will forever be remembered throughout history as the greatest builders of ancient times.
Chevallier, Raymond. Roman Roads. Berkley and Los Angeles: University of California Press, 1976
Dilke, O. A. W. The Roman Land Surveyors - An Introduction to the Agrimensores. New York: Barnes & Noble, Inc., 1971
Dilke, O. A. W. Greek & Roman Maps. Baltimore: John Hopkins University Press, 1998
Hall, Michael and David Phegley. Home page. 1998. "Ancient Roads of the Romans" www.geocities.com/Athens/Academy/5769
Hamey, L. A. The Roman Engineers. Minneapolis: Lerner Publications Company, 1982.
Lewis, M. J. T. Surveying Instruments of Greece and Rome. Cambridge: New York: Cambridge University Press, 2001
Nardo, Don. Roman roads and aqueducts. San Diego: Lucent Books, 2001
Neill, Mike. Home Page. "Roman Tables". March 9, 2001. "Measures of length". www.romantables.8m.com/length.html
Stone, Edward Noble. Roman Surveying Instruments. Seattle: University of Washington Press, 1928
 Hall, Michael and David Phegley. Home page. 1998. "Ancient Roads of the Romans" www.geocities.com/Athens/Academy/5769
 Nardo, Don. Roman roads and aqueducts. San Diego: Lucent Books, 2001. Ch. 1, Pg. 13
 Nardo, Don. Roman roads and aqueducts. San Diego: Lucent Books, 2001. Ch. 3, Pg. 41
 Dilke, O. A. W. The Roman Land Surveyors - An Introduction to the Agrimensores. New York: Barnes & Noble, Inc., 1971. Ch. 1, Pg. 15
 Neill, Mike. Home Page. "Roman Tables". March 9, 2001. "Measures of length". www.romantables.8m.com/length.html
 Stone, Edward Noble. Roman Surveying Instruments. Seattle: University of Washington Press, 1928. Vol. 4, No. 4, Pg. 240
 Dilke, O. A. W. The Roman Land Surveyors - An Introduction to the Agrimensores. New York: Barnes & Noble, Inc., 1971. Ch. 5, Pg. 73
 Stone, Edward Noble. Roman Surveying Instruments. Seattle: University of Washington Press, 1928. Vol. 4, No. 4, Pg. 219
 Hamey, L. A. The Roman Engineers. Minneapolis: Lerner Publications Company, 1982., Pg. 27