Southern Polytechnic State University

Team Members

Students -- Blake Prince, Jack Mills, Jerry Wood, Jeremy Shirey

Faculty Advisor -- Matt Wilson

Southern Polytechnic State University - Civil Engineering Technology Department

Throughout history, land surveying has been an integral part to the growth and prosperity of a culture. The Romans used surveying instruments to layout roads, as well as to layout the grids on which cities were built. The surveying instrument that the Romans used was called a groma.

The word groma is thought to have come from the root Greek word gnome, which is defined as the pointer of a sundial. The true origin of the word is unknown. The groma itself is an instrument used to turn right angles. The importance of turning right angles is shown in the layout of several cities in northern Italy as well as North Africa. The land was sub-divided into 2400 Roman feet square. Each square was called a centuria. The centurias were crossed by road at right angles to each other. The roads that ran north-south were called cardines, and the roads that ran east-west were called decumani. The groma was also used to locate and place monumentation for property corners. Once the centurias were in place, maps were drawn on bronze tablets giving a physical representation of the centuria.

The groma is most commonly known for its use in the layout of roman roads. Roman roads are known for their straightness, as well as their durability. Some Roman roads still stand as a testament to the ingenuity of the Roman engineers. The groma was sued to stake out straight lines between two intervisible points. Once the surveyor had his line of sight he would then have his assistant place poles on this alignment. The groma could also be used to put roads 90 degrees to each other. This was done by trial and error. The gromas would be set in line with one alignment, the surveyor would sight down the cords, and he would then sight down the opposite cords. The surveyor would keep moving the groma until he was able to sight down both alignments.

Much like the origin of the word groma is a mystery, so is the use and the shape of the groma. There are several known fact about the groma as well as several points that could be argued. The groma clearly consisted of two main parts, the ferramentum or "iron" that was used by planting it in the ground, and the main part of the groma, which was placed on top. The main part of the groma consisted of a cross with four arms. The cross is thought to be 46 cm long wooden arms with iron sheathes. At the end of the arms of the cross were plumb bob cords. At the end of the cords were plumb bobs. The center of the cross was called umbilicus soli. The cross could pivot on a point at the end of an offset bracket. The offset bracket could also pivot on a pole extending from the ferramentum. The offset bracket is thought to be made up of two wooden brackets 25 cm long with two strips of bronze to reinforce them. The actual size and shape of the groma has just recently become a topic of argument.

The way that the groma was used is still unknown. It is thought that the surveyor sighted diagonally across opposite cords at rods placed on line by his assistant. If a new alignment was going to be established, the point directly below the umbilicus soli was marked using a plumb bob and the surveyor would sight down the cords 90 degrees to the ones he was previously looking down.

The groma had three very large limitations to its accuracy. First, the groma's plumb-lines were very susceptible to movement by the wind. Protecting the plumb-lines from the wind was unfeasible. The groma was also only accurate at very short distances in good weather. Second, it was very hard to sight using the cords. The cord that was closest to the surveyor appeared much larger than the cord that he was sighting on, which caused difficulty centering the second cord. Third, the groma was only able to survey in the horizontal. To traverse up and down hills the surveyor did what's known as cultellatio.

Cultellatio is performed by holding a ten-foot rod horizontal to the ground. The surveyor used his eye to hold the rod horizontal. With one end resting on the ground and the other end in the air, a plumb-line was dropped from the elevated end. The pint was marked where the plumb bob hit the ground. The rod was then moved, but it was kept horizontal to the point where the plumb bob hit. This procedure was followed until the surveyor had traversed the complete hill.

Many of these surveying techniques are thought to have come from Greece. There are several similarities between roman surveying and Greek techniques. The Corpus Agrimensorum is a manual of Roman surveying practices. The manual was written in the fourth century A.D., and offers one of the only looks into the surveying techniques employed by the Romans. The manuals text is incomplete as well as being very obscure. The manual does not discuss the groma. In the manual, there are several similarities to Greek surveying methods. One such similarity is the way in which the Romans measured the width of a river. The Romans used similar triangles, which is a Greek practice. In the Corpus Agrimensorum the procedure described to measure the rivers width is almost verbatim to the same procedure in Greece text.

The roman surveyors with their ingenuity and instruments helped pave the way for modern surveyors. Although the technologies have improved, many of the same basic concepts that the roman surveyors used are still being used today. Over time, the only thing that changes in surveying is the equipment and techniques. The theory and concepts are a constant. This constant was started with roman surveying. With all of the advances in surveying over the year, it is easy to lose sight of what surveying is really about and basic concepts that it uses. Looking back at roman surveying provides a greater appreciation for the surveying practice and all that it encompasses.

Lewis, M. J. T. Surveying Instruments of Greece and Rome 2001, Cambridge University Press