The Upright: a one-and-a-half to two-metre-long pole. One end was metal-tipped to facilitate pushing it into the ground.
The Rostro: an upwardly-curving right-angled extension, with an end fixed to the top of the upright via a swivel mount. This extension rotated horizontally through 360 degrees. The centre-point of the groma was attached horizontally to the upper vertical end of this extension via a swivel mounting. A plumb line was fixed beneath the groma's mounting point to act as a third sighting point.
The Groma: a simple cross of wood with arms of equal length. Each arm had a plumb line suspended from the end of it; each line was equal in length to the other three plumb lines.
The Marker Peg: a peg positioned in the ground below the groma-mounting plumb line to mark the datum point.
How It Worked
When the groma was set up, generally on a high point or rise in the ground, the surveyor would turn the groma (the cross) so that one of the arms pointed in the direction that the road or construction was to take.
The surveyor would send out an assistant with some ranging poles; the assistant would stop after about 125 paces and hold a pole vertically with one end on the ground. The assistant was directed to move until the surveyor could see that the pole was in line with the three strings of the groma: the two on opposite arms of the cross (north and south) and the plumb line fixed to the groma end of the rostro. The process was repeated until the assistant had run out of poles, leaving a straight line of poles marking the course of the road. The surveyor would then move forward to the last pole, set up the groma again and send out his assistant with ranging poles to repeat the procedure.
If laying out a right-angle or corner of a building, the groma's east and west plumb lines would be used.
Who Used The Groma
Gromatici: general building and road surveyors.
Agrimensores: property and land-surveyors.
There were four main specialist types of Roman surveying;
- Military surveyors laid out the roads throughout the Empire. They were also responsible for surveying the sites for marching, camps, forts and fortifications. These men also laid out the positions of Hadrian's Wall and the Antoine Wall.
- Agricultural surveyors were employed to supervise the allocation of farmland in the new colonies and to supervise the distribution of lands around the town in which the surveyors were based. Thanks to the groma, it is no wonder that early Roman field systems were noted for their square or rectangular shape.
- Architectural surveyors laid out the municipal buildings and streets of the towns and cities.
- Services surveyors, often working with architectural surveyors, were responsible for works such as water supply, and were controlled by the appropriate urban surveyor.
Surveying With A Groma
The roads were surveyed in the same way throughout the Roman world. It was impossible to lay out a perfectly straight road over many miles using the groma. This explains the course corrections (slight bends) that occur every few miles2.
Groups of surveyors were used when a road was built between two distant points.
Firstly, the route was surveyed in sections marking out the high points. Then, after the first stage had been completed, the route was surveyed again between the high points to make it as straight as possible.