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Team
Members
Students
-- Christopher Jones, Adam Salazar, Monica Rivera, Mark Marrujo, Rudy
Keedah, Marvin Murphy
Faculty
Advisor -- Earl F. Burkholder |
ROMAN
ERA SURVEYING
Compiled
by:
New
Mexico State University - Roman Surveying Team
Abstract
The National Society of Professional Surveyors (NSPS) has created a surveying
student team competition to be held at the joint American Congress on Surveying
and Mapping (ACSM); American Society on Photogrammetry and Remote Sensing (ASPRS);
and Federation Internationale de Geometres (FIG) annual spring conference in
Washington, D.C. on April 23, 2002. In accordance with the guidelines
directed by NSPS, we (a team of six surveying engineering students from New
Mexico State University) have reproduced two roman surveying instruments:
the Groma, and the Libra. The following report entails the documentation
of the research conducted and surmised to create the aforementioned roman
surveying instruments.
The military and monetary power of the Roman Empire enables their society to
build an infrastructure of roads and water systems responsible for the
sustenance and propagation of a society lasting hundreds of years. At the
heart of roman civil engineering projects was the Roman surveyor. He was
an integral part of the building of this great empire. Unfortunately, the
written history of the methods and tools of the Roman surveyors have long since
been lost with time. There are no field books or explicit manuals
explaining the daily computations and surveying techniques of the Roman
surveyor. In addition, there is little archeological evidence of the use
of properties of surveying instruments used by the Roman surveyors. Much
of the modern day understanding of how the Romans surveyed is based on
conjectures from a small amount of evidence.
Archaeologists and historians have had to piece together their own versions of
Roman era survey instruments by speculating on the various metal part found in
archaeological sites of Pompeii and the rest of the former Roman Empire.
The only verifiable depiction of surveying instruments has been found on
tombstones like that of Lucius Aebutuis Faustus. The Corpus, a
collection of manuscripts from Roman surveyors assembled into a manual of survey
instructions provides insight into the practice of surveying, but does not
describe the instruments used. The writings of Heron of Alexandria
describe a wonderful multi-axis surveying instrument called the dioptra,
but ther is no evidence to suggest that the Roman surveyor adopted its use.
At one known excavation in Bavaria, an iron cross was discovered with a hole at
the intersection of the arms. At the end of the four sections, there were
holes with the remains of iron nails. It is speculated that this
particular artifact had a wood frame around the cross as well as a wood base and
the iron nails were used to hang a plumb lines. This find is the subject
of debate as to whether or not it is an instrument; some contend it to be a groma
while others think it an agricultural implement. In 1912, on an excavation
in Pompeii, Della Corte discovered the office of a Roman surveyor named Verus.
The metal parts of a groma were found along with other instruments such
as poles and measuring rods. Della Corte's reconstruction of the groma has
become the standard image of what this type of instrument looked like.
Recent research has disclosed that the parts Della Corte assembled into his
instrument were found in several different locations, suggesting that his
reconstruction is not a very accurate one.
What is known from these sources is that the groma was the most common
instrument used during the Roman period and it served as the primary surveying
instrument used by the Roman surveyor. The design and principle of the groma
is straightforward. The groma consisted of 2 arms
(approximately 3 feet in length) crossed at the middel, one being perpendicular
to the other. These arms were attached to a swinging bracket. This
bracket was attached to a wooden staff offsetting the groma for greater
ease of use. At the foot of the staff was the ferramentum, an
"iron foot" having four large flukes or wings for stability purposes,
and a sharp tip on the end of it. This served as the base for the
staff. From the end of each of the four arms hung a plumb bob or a
cord. There was a fifth plumb bob that was attached to the, umbilicus
soli, the center of the crossed arms. This was used to located the
instrument over the point from which observations were to be made.
The groma was limited to sighting, setting out straight lines, and right
angles. The surveyor worked only in the horizontal plane with this
instrument and was not concerned with differences of height. The surveyor
sighted diagonally across one pair of cords to project a straight line or to set
a perpendicular line to the line defined by the other pair of plumb bobs.
This simple instrument was well suited to surveying road alignments,
establishing the gridwork of roads and streets for towns and military camps, and
above all, for rectangular land subdivisions. The one shortcoming due to
the groma's simplicity was that wind played havoc with the plumb-bob
strings, making it very difficult to survey on blustery days.
Leveling instruments are not mentioned except by name in any of the books of the
Corpus. The name libra aquaria appears several times
throughout the texts. This has traditionally been interpreted as being a
water level, some instrument filled with water to define a level surface.
A recent approach has interpreted this term as a level for laying out
watercourses, specifically for aqueducts. Another form of level called the
chorobates, meaning "land walker", is well described in
Vitruvius' (an architect) text. This was an instrument 20 feet lone, with
plumb line levels at each end and a groove along the upper surface to be filled
with water. It is not mentioned in the surveyor's texts and appears better
suited to maintaining grade during construction rather than for differential
leveling.
The Egyptians and Greeks of the era are known to have used a rope marked off in
increments to measure distances. The Roman surveyor apparently did
not. There are references in the Corpus of using cords for
alignment while measuring, but not for the act of determining the
distance. What the Romans used were ten feet long wooden rods with metal
ends. These rods, called pertica, were laid end to end along a
course of survey to determine the total distance. While much slower, this
method was more accurate than the use of the rope, because the poles did not
give erroneous measurements due to stretching and contracting. Specialized
methods for measuring horizontal distances were developed. Steep sided
valleys too wide to measure across in the ordinary way were measured by an
operation known as cultellatio. The surveyor would hold a ten-foot
rod horizontal with one end resting on the ground at the beginning of the
slope. At the other end of the rod the surveyor would drop a plumb line
and mark a point where it hit the ground. He then moved the end of the rod
to this point still horizontal and repeated the first step continually going
down the slope and up the other side to the point of ending, measuring the
horizontal distance.
The Romans used a somewhat different unit of measure than that of modern
times. The Roman foot, which is 11.65 modern inches, was divided into 16
inches rather than the modern division of 12 inches. The Roman foot
consisted of four palms of four Roman inches, where a palm is about 3 modern
inches. The Roman mile consisted of 1000 paces, or 5000 Roman feet.
There are four values of feet and stades, which were used, quite often in
Hellenistic and Roman times:
|
Attic-Roman |
Standard |
Olympic |
Philetaeran |
1 foot |
29.6
cm |
30.8
cm |
32.0
cm |
32.9
cm |
1 stade |
177.6
m |
185 m |
192 m |
197.3
m |
stades
to Roman mile |
8.33 |
8.00 |
7.71 |
7.50 |
|
|
|
|
|
|
Roman |
|
|
|
16
digits |
1
foot |
12
inches |
|
|
24
digits |
1
cubit |
|
|
|
5 feet |
1
pace |
|
|
|
1000
paces |
1
mile |
|
|
|
When performing layouts, the square was
the most common area. The smallest square area was known as a scripulum,
which was 10 Roman feet by 10 Roman feet. The next largest area was known
as an acnua, which was made up of 120 Roman feet by 120 Roman feet, or 1 actus
by 1 actus. An iugerum was rectangular area comprised of acnuas,
and then a heredium was simply 2 iugerum. The largest square
was defined as a centuria, which is 2,400 feet by 2,400 feet, or 20 actus
by 20 actus. This is comprised of 10 heredium by 10 heredium.
Given the above research, we
determined to recreate enough Roman era survey instruments to be able to perform
leveling, lay out right angles, prolong a line and measure distances. We
selected the groma, the libra and the pertica as being
capable of performing the tasks presented at competition. We also accepted
the Roman survey foot as 29.6 centimeters and constructed all instruments to
dimensions in those units. In keeping with the technology of the time, all
iron parts have been hand forged, the wood pieces finished with linseed oil, and
the plumb-bobs cast from lead (plumbium is Latin for lead, hence the term
"plumb line").
The design of the measuring rods was essentially a given; two wooden rods ten
Roman feet long with iron tips, graduated in even feet except at each end where
the first/last foot was split into 16 digits. A spool of twine is used to
provide alignment while measuring longer distances. The other instruments
are based upon our interpretation of the evidence found.
Recognizing that the authors of Roman archaeological history texts have not been
surveyors, and the difficulties associated with translating Old Latin into
modern English, we have made some unique interpretations of our own. The Corpus
makes mention of the groma in text, but more often refers to its base,
the ferramentum, when describing its use in survey. We decided that
the ferramentum was analogous to our modern tripod, being the foundation
for the instruments that level and provide alignment. Therefore it had to
be short enough to sight over so we built it at five feet tall instead of Della
Corte's suggested two meters. The actual point on the lower end is of four
blades to provide stability like the base that Della Corte did find. The
upper end has a threaded pin that will accept both the groma and the libra.
A plumb line is built into the shaft to indicate verticality.
The arms of our groma are about three feet long, with lead bobs at each
end suspended by plant fiber strings. The instrument is set on an offset
hinged bracket with another suspended bob that is used to assist in setting up
the instrument directly over a point or line.
Our libra is six feet long with wooden slits for sights. It
balances on the threaded pin atop the ferramentum and is weighted for
stability with a matched set of plumb bobs. One of the pertica is
used as a leveling rod, but with the addition of a movable target. The
instrument operator provides alignment, while the rod person takes the
readings. To assist the survey during windy days, a screen of cloth is
used to shield both the libra and groma from the disturbing
currents of air.
BIBLIOGRAPHY
Roman
Land Surveyors O.A.W. Dilke' Newton. Abbot, David & Charles,
1971
Surveying
Instruments: their history E. R. Keily. Columbus, Ohio,
Carben Surveying Reprints, 1979
Greek
and Roman Technology K. D. White. Ithaca, New York, Cornell
University Press, 1984
The
Writings of the Roman Land Surveyors - Introduction, Text, Translation and
Commentary Brian Campbell. Great Britain, Stephen Austin &
Sons Ltd., 2000
Surveying
Instruments of Greece and Rome M. J. T. Lewis. Cambridge, UK,
Cambridge University Press, 2001
Antepasados
Surveyors in History Wilfried E. Roeder, P.S. Albuquerque,
New Mexico, New Mexico Professional Surveyors, 1995
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