Week 9: Groma Lab Summary

By: Amalia Pappa, Luisa Cichowski, and Soren Eversoll

Featured image: sketch by Em Jahn


In our Experimental Archeology lab last week, we learned how to use an ancient Roman measuring tool called a Groma to perform a variety of surveying tasks.  The archeological record shows that ancient Romans were very adept at creating straight lines and right angles in order to carry out their architectural projects.  We recreated a simple but functional version of the tool they used as the basis for these measurements in order to learn more about the ancient Roman surveying process.

In preparation for the lab, each group A-F built their own simple Groma using modern materials (see ‘Procedure’ for more details).  In the lab, the groups used the Gromas to perform three historically relevant tasks (measuring roads, a templum, and centuriating a patch of land), using their observations on the experience to answer questions about how ancient Romans accomplished their highly accurate feats of measurement.

The data which this lab was able to provide was mostly experiential and qualitative.  Some questions guiding our inquiry were: was the simple mechanism of a groma sufficient to perform the tasks of a measuring road, measuring a templum, and centuriating land? What was it like to use a groma to perform each of these tasks? Were there unforeseen challenges to using a groma, and what skills did this process rely heavily on? What is the learning curve for using a groma to perform each of these tasks?

Group F using the groma to create their templa. Photo by Isabel


The Gromas were constructed in class using cut-up PVC pipe, string, and screws attached to the bottom of each string that weighed them down. One long, upside-down L shaped piece of PVC pipe served as the stand for the Groma. Another much smaller cross made of PVC pipe was attached to the end of this stand. On the end of each side of the PVC cross, string was wound through a hole. A screw was then tied to the end of each string. These weighted strings stood at about half of the length of the PVC stand.  Another, longer string was then wound through the center of the cross and a screw was attached to it. This line would help groups to line up the Groma with the marker flags placed in the ground.

On the day of the lab, groups A, B, and D were first assigned to make a road, laying out a path from the Mini Bald Spot as far as they could go before being called back by Jake. As they were doing this, groups C, E, and F were assigned to create the foundation for a templum, a Roman sanctuary, with sides ranging between 20 and 30 feet with right angle corners. Then, upon successful completion, they constructed a templum with sides doubled in length. These groups then switched roles.

After this was done, the groups were broken up and reformed into larger sections and then tasked with centuriating (a method of Roman land measurement done by marking up land into an even grid), the Mini Bald Spot and the Midi Bald Spot (the patch of land across from Anderson). Groups subdivided an area using the Groma, flags, and a measuring wheel using dimensions provided by Jake on the day of the lab.


Making A Templum

Almost every group failed at making a perfect templum, a square space. Most groups struggled with creating lines of equal length as the angles created with their groma would be just a bit greater than or less than 90 degrees. Despite not making perfect squares, each group succeeded in making a quadrilateral that closely resembled a square. It seems that the groups who did the templum first (C, E, and F) had a much more difficult time than those who did the road-building first (A, B, and D) and then the templum. However, almost all of the groups agreed that the templum was much more difficult to do than the road-building task. Much of this evidence speaks to the learning curve of the groma. The groups that started with road-building were able to spend more time with the groma before starting on their templum. A possible explanation for these groups seeming to have less difficulty than the others with their templa is that the learning curve must be pretty steep. The groups who did the templum second had, on average, only been with the groma for 45 minutes. However, it is important to note that despite having an easier time, this group still struggled in making perfect squares. Finally, most groups seemed to find making straight lines with the groma a lot easier than making 90-degree angles with the tool.

The directions in the manual were simply to “lay out a templum of sides between 20-30 feet with right angles at the corners.” Each group went about measuring these lines in different ways, and some had greater success than others. In the table below is a summation of each group’s procedures and the sizes of the templum they were able to make:

GroupProcedureTempla Size
ANo procedure provided however they attribute their last side being too long to an incorrect measurement in at least one of the anglesSmall: 35×35 (4th side 2 meters extra)
Large: 66×66
(4th side 4 meters extra)
C“we first laid out two straight lines meeting in a right angle, then laid a third side of the box and measured it to the length of the first two. Having done that, we now had four corners”Small: 33×33
Large: 66×66
DUsed “the measuring wheel to calculate the lengths of the sides of our squares” then moved “from established corner to established corner”Small: approx. 10×10
Large: approx. 23×23
E1st templa: “struggled to implement any real procedure to measure out the lengths of each side.”
2nd templa: “1) Measuring the approximate desired distance with the measuring wheel from the zero point 2) Align the pole with the Groma’s strings 3) Re-measure the distance and re-align as necessary.”
Small: approx. 7×7
Large: 14×14
F1st templa: measured lines by placing a flag at either point and then eyeballing a straight line between the two with the click wheel
2nd templa: used string to draw out a line between the two points and then measured along the string
Small: 30×30
Large 60×60

Building a Road

Among the lab’s simpler tasks was using a groma to build a “road.”  Their goal was to begin on the Mini Bald Spot and then see how far they could go using the ancient method. First a Groma’s position was established, marked by the central weight hanging over a flag. Next, two group members with stakes and flags walked ahead of the Groma and were instructed where to stand by the Groma operators, the Agrimensores, who twisted the cross so that it aligned with the direction they were heading. One Agrimensore held the Groma in place and made sure that it was directly over the marker flag. The other looked down the plumb lines established by the cross and instructed two other group members were to stand so that their positioning aligned with the hanging plumb lines. This was done by looking down the lines and squinting one eye to maintain a fixed perspective. Once the Agrimensores were satisfied with the positioning, two flags were placed in their spots (the stake-holders were reasonably separated throughout this process). Next, the Groma advanced to the first established flag, the new marker flag, and the process was continued by adding two more flags down the line, advancing flag by flag. If large obstacle such as hills and trees were encountered, the Groma line pivoted into a right angle. 

Every group went about the task slightly differently; the table below shows the length of road each group measured, how long it took them to measure it, and any additional descriptions about the road’s features, if the group decided to record them:

Length, time to measure, and features of roads by group:

GroupLength of RoadTime to Measure RoadFeatures if noted
A“From the north-most tree on the field outside Anderson Hall to the grass outside Gould Library”30 minsthree right angles and crossed over two sidewalks
Capprox. 1,000 ft45 mins
D176 ft and 2 inches45 minsStayed within mini bald spot field
E520 ft, or “the southeast side of the Mini Bald Spot to the northeast corner of Myers dorm”40 minsThree continuous straight portions with two turns at various obstacles
FApprox. 750 ft, or “started at the Mini Bald Spot and made our way to the closer edge of the Bald Spot”30 mins

A few of the groups also described their process for using the groma to measure a road.  Although all groups were instructed to begin using the groma in the same manner, over time and throughout each activity groups adjusted their method for effectiveness. This gives us some insight into the way in which ancient Romans may have chosen to use gromas to measure roads.

Process of using the groma to measure a road by group:

GroupNotes on Process
A“We were able to use the groma to easily pass over [the] areas of cement.”
D“Since a physical connection between each point in the line is not needed, as it would be if we were perhaps laying out our road using string instead, any obstacle could be avoided by simply placing the groma and poles on opposite sides of it, or, alternately, by moving around it in right-angled increments so as to reconnect with the original path of the road.”
E“In terms of our process, we would measure 2-3 poles from each zero point.”
F“After drawing the first couple of points, we landed on a process which involved one person standing at the previous point with a pole, and another standing at the site of the future point so that the person in the middle with the Groma could line both points up with one another.”
timelapse of the distance Group E covered with their road. Video by Dylan

Finally, some of the groups recorded qualitative descriptions of their experiences building a road and the challenges they faced.  For instance, Group E talked about how before they began the road building process, they chose to elongate the center string of their groma in order to make it easier to align the groma’s zero point to the marker (flag in the ground) from which they were measuring.  This shows that the center string of a groma was likely always longer than the other strings, as this helps keep measurement consistent.  Group F noted that building a road was “considerably easier” than building a templum, because they did not need to measure angles to such accuracy.  This observation shows that a lot of the expertise of using a groma is needed to measure angles, while measuring straight lines has a quicker learning curve.  Group E echoed this reflection, saying that “the road-building process proved to be far less intense and mentally taxing than laying out the templum.”  They also noted that building a road felt more relaxing and allowed them to connect with the campus environment in a way that was fun and “very welcome.”


For centuriation, groups on the Mini Bald Spot and the Midi Bald Spot were tasked with dividing an open space into even rectangles, as the Romans did to portion out land in their settlements. This served as the amalgamation of the skills learned in the templa and road experiments, requiring both long, straight lines, measurement, and right corners. The tools required were a measuring wheel, stakes, and the flags, as well as the Groma (obviously). In the case of the Mini Bald Spot the land was divided into a 4 by 4 grid of 16 sections, ultimately 88 by 88 feet. Each individual square was thus 22 by 22 feet (22 feet = 264 inches). Much like in the construction of the road, the Gromas were used to plot out straight lines using the staggered, flagged marking system. Once this had been established, the 2 sides were divided into 4 equal sections using the measuring wheel. To aid their accuracy in this process the group devised to place string between the flags, connecting the parallel lines of the centuriated space. The meeting points of these lines designated the corners of the smaller, 22 foot sections. This, the Mini Bald Spot group noted in their data reporting, would not have been possible for the Romans, who were plotting out much larger swaths of land.

The group on the Midi Bald Spot plotted out an area that was 60 by 64 feet, a little more confined than the space the group had on the Mini. This space was divided into four squares that were 30 by 32 feet (30 feet = 360 inches; 32 feet = 384 inches) each. At this point in the lab the sun had been beating down and morale was low. Additionally, this group was too large and had to split off into a smaller group. Groups of four subdivided themselves, each drawing a straight line. These lines connected to make the first corner of the centuriated space. They split up the measurements on each side, half and half, adding them up to reach their aggregate goal. After the lengths of these three sides were determined, the group measured and identified the meeting point of the ultimate square.

Findings and Conclusion

Using the data we provided, we can now seek to answer the questions presented in our introduction. First, was the simple mechanism of a groma sufficient to perform the tasks of measuring a road, measuring a templum and centuriating land? Our data shows that once we became familiar with the groma, creating a road was a relatively easy feat. The groma was a great tool for creating straight lines along a great distance, given that at least four people were involved in the operation. However, the groma seemed to be relatively ineffective in creating a templum. While the angles constructed with the groma were nearly 90 degrees, it appears that none were perfectly 90 degrees. If we needed all of our sides to be of equal length with corners of 90 degrees, as we did with our templum, the groma would not provide the accuracy necessary. When centuriating the land, the groma was extremely helpful in creating the initial large rectangle. However, it seemed to be less successful in the construction of the grid. Most groups seemed to have ditched the groma altogether, using only the string and the clicking wheel to make the grid.

In terms of the challenges and skills required of the groma, there seemed to be plenty of both. Almost every group recorded a mishap with their groma. Group C mentioned that their “weighted cords swung too much,” Group E had to elongate their center string to more easily determine their center point, and the arm of Group F’s groma fell off a couple of times and their cross piece was slightly tilted. On such a windy day, utilizing the groma required a significant amount of patience as we all had to wait for our strings to stop moving so that we could align them properly. Maybe, on a less windy day, our problems would have dissipated immensely, but ancient Romans had windy days too! Of the physical skills required, many people found it more difficult than expected to align the poles with the groma strings using their eyes. Of the social skills required, communication was essential however some groups noted that it became more difficult to communicate as the experiment progressed and people became tired.

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