Roads and surveying
Blackstone Edge Roman road
In order to move the army quickly across the country, the Romans built tremendous roads. I had always been told that Roman roads are straight, but I really had this rubbed in on the A68 north of Corbridge, where the road goes in a dead straight line for miles over the rolling hills. In fact it's rather dangerous for driving; because it is so straight you are tempted to drive fast. Nothing in the Roman world did more than say 20 m.p.h. (a galloping horse) and they built their roads up and down steep hills - sometimes as much as 1 in 6.
The result is that not only are there some fierce hills to climb, but often there are blind summits, where you can't see oncoming traffic even 50 yards away. So if you drive too fast and try to overtake you are liable to meet someone else coming the other way.
Building roads in a straight line is not difficult - you merely have to plant two canes in the ground, walk ahead and plant a third in line with the first two, and so on. Sighting along canes gives good straight lines for miles. However, what really impressed me was how they managed to set off in the right direction. For example, when the Romans wanted to build a road from London to Chichester, they knew exactly where to head for, even though the distance is 65 miles, there are several hills in the way and they had neither maps nor compasses.
There's an old saying 'I hear and I forget; I see and I remember; I do and I understand.' Roman road surveying was just like this for me. I learned and understood how to do it by surveying a route over high sand dunes from one flag to another that I could not see. I knew roughly which way to go; so I went to the high point on a sand dune not far from the route and put a beacon there. Then I walked on and planted another beacon at the next high point, from where I could see the goal. I went back to the first beacon and moved it to the straight line between the first flag and the second beacon by using a groma.
The groma was the standard Roman surveyor's instrument. It's an upright stick with a couple of bits of wood fixed to the top to make a cross. From each end of the cross hangs a little weight on a string. When the groma is stuck in the ground you stand behind it and twist it until you can sight along two of the strings to the starting point. Then you walk around the groma and sight the other way, to the second beacon. If the strings do not line up with the beacon then you move the beacon beside you in order to get more in line. Plant the beacon, plant the groma, and try again, until the strings line up with the start point in one direction and with the second beacon in the other. Then you know that the start point and the first two beacons are all in one line.
Repeat the whole process with the second beacon, then with the first again, and the second again, until the start point, the finish point, and both beacons are in the same line. This process would be much more efficient if one surveyor were standing at each beacon, ready to move it - and even better if they all had mobile phones!