When the Romans started to build a road, they most often couldn’t see the destination they were heading for, yet they still managed to plot a straight route between the two, so how did they do this? The answer is that they used a simple but effective combination of beacons and a surveying instrument called a ‘groma’. This chapter includes a video, a text and an infographic explanation of the surveying method used by the Romans to build their roads.
How to use a groma
Step 1: Using Beacons
Standing in Londinium the Roman surveyor knew the general direction of Noviomagus, the city he wanted to build a road to, but he couldn’t see Noviomagus from where he was standing because there were hills in between.
The surveyor would therefore light a beacon in Londinium and then set off in the general direction of Noviomagus. When he got to the highest point in the landscape where he could still the beacon in Londinium, he would place another beacon. He would then set off again, in the general direction of Noviomagus, placing beacons along the way and always making sure the beacons were on a high point in the landscape, and that he could see the one he placed before when placing the next one. Eventually, the surveyor would be able to see Noviomagus and place the last beacon in the town.
There would now be a line of beacons across the countryside linking Londinium with Noviomagus, although they wouldn’t be in a straight line. Getting the beacons to align is where the groma came in.
Step 2: Using a Groma
A groma was a simple but effective instrument and every Roman surveyor would have one. It consisted of a long stick with a cross of two more sticks fixed at right angles to each other mounted on top. From the end of each arm of the cross hung a string with a weight on it, making a plumb line. To use it, the groma would be pushed into the ground so that it was upright and the plumb lines hung vertically.
Lining up the beacons
The surveyor would put the groma in the ground at the site of the last beacon before Noviomagus and line up two of the groma’s plumb lines with the beacon in the town of Noviomagus. Then he would walk around the groma without touching it, to see if the other two plumb lines lined up with the previous beacon. If the previous beacon was out of line with the plumb lines, it would be shifted to place it so that it was in line. This was much easier and quicker to do with a person on each beacon, but remember, communication over distances was limited! In forests, fires would be lit and the smoke used to help line up the beacons. In this way, the surveyor could place the groma in one location and get three beacons lined up – the one in Noviomagus, the one he was standing at, and the next one towards Londinium.
It was then a case of moving between the beacons, using the groma to line each one up with the beacon before it, and the one after it, shifting the beacons one way or the other until eventually they were all in a straight line between Londinium and Noviomagus.
It would have been a long and tedious job, but the resulting straight roads were a major factor in the ability of the Romans to keep control of Britain and to export the island’s raw materials.
The Romans needed straight, wide, solid roads to transport troops and goods. The roads they built were made from aggregates – lots of different sized stones that compacted down to create a strong, stable and long-lasting surface. These roads could stand up to the marching of hundreds of soldiers, and carts laden with supplies.
The Romans brought two more road building innovations to Britain; road camber and drainage. Romans built their roads with a camber, or a slight convex curve to the surface, which we still use today. This prevents water from sitting on the surface of the road making it soggy, instead channeling it to the edge of the road. Coupled with this, the Romans built drains alongside the roads in order to absorb this runoff from the cambered road and channel it away.
The photograph below shows a cross-section of a modern reconstruction of a Roman road. The reconstruction is built on top of the land and uses metal ‘cages’ to keep the aggregate in place for the purpose of showing how the roads were made. In reality, the road would have been dug into the land and constructed without using wire ‘cages’.
Along the route of their roads, the Romans placed carved milestones as signposts and distance indicators to other Roman settlements.