A groma was a Roman surveying instrument: a horizontal cross with weighted plumb lines hanging from its arms, mounted on a staff. Surveyors sighted along the lines to set out straight lines and right angles for roads and land.
Key takeaways
- A groma is a cross with plumb lines at the ends of its arms, held on a vertical staff.
- Surveyors sighted along the plumb lines to mark straight lines and right angles on the ground.
- It worked best on level ground or gentle slopes, and wind could disturb the plumb lines.
- Our map shows where roads ran, such as the 93.3 km of Stane Street, not the tools used to lay them out.
How a groma works
A groma was a Roman surveying instrument. It let surveyors lay out right angles and straight lines, which also made it possible to divide land into a rectangular grid [1]. It is the only Roman surveying tool of which examples survive [1].
The instrument was a horizontal cross with a plumb bob hanging from the end of each arm, set on a vertical staff. The centre of the cross was the reference point. The surveyor placed that point over a marker and turned the cross to the direction he wanted. Sighting through strings on opposite arms, he lined up poles that an assistant placed on the ground. Distances were measured with rods [1].
For road building, surveyors used rods to sight a straight line and the groma to set right angles [2]. See why Roman roads are straight for how that fed into route choice.
Its limits are worth knowing. The plumb lines are sensitive to wind. The far plumb line can look thinner than the near one, which causes a small angular error. Scholars also say the setup suits level ground or gentle slopes, and how surveyors dealt with steep valleys is unknown [1]. Much of its detailed use is debated, so reconstructions in museums are best treated as informed guesses.
An example from our data
Stane Street runs for 93.3 km in our data, in 8 segments, and its page shows the route. Ermine Street is 421 km long. Both have long straight runs that were laid out by sighting between points, and a groma is the kind of tool that would have been used in setting out such lines. Our dataset has no record of the tools, the surveyors or the setting out, only the resulting route and its certainty. Of Stane Street, 99.6% is conjectured and none is certain, so the line on the map is an estimate.
The thing people get wrong
A groma is not a general surveying instrument for everything. It fixed lines and right angles. It could not measure levels or heights, and it does not explain how roads climbed hills or crossed valleys. Another common mistake is to say the Romans measured a whole road at once with it. Surveyors worked in stages along the line, and the final route also depended on terrain, rivers and older tracks, not on a single instrument.
Frequently asked questions
What was a groma used for?
Setting out straight lines and right angles, for roads and for dividing land into rectangular plots.
Did the groma measure heights or slopes?
No. It was for lines and right angles on the ground, and it worked best on level or gently sloping land.
Sources
- Groma (surveying), Wikipedia.
- Roman roads, Wikipedia.
- Every figure from our own map is worked out from the datasets on the data sources page (checked against Itiner-e version 2024 1.3).