Turns out they did it by measuring in baby steps, sighting along a level plane at a measuring stick, a bit further up or down the grade. Kind of like this:
In order to shoot a perfectly horizontal line, they needed a tool, so to help them out, I set about inventing one. The first thing that came to mind was of course water, which is always level (found out later that they did have one tool that involved sighting over a pan of water, but I didn't know that and besides, I think mine is more elegant..)
 |
| Gary's Ancient Leveling Transit |
Here's my prototype:
Since I didn't have an assistant, I propped a measuring stick up at a slightly different elevation and taped yardsticks to it.
First, I calibrated the stick by placing it right next to the transit and marking it at the same height as the nail points. Due to uneven density of the wood, one point was a little higher than the other when I spun it around to check against the first mark, so I added a Roman coin (actually a washer) to balance it out.
 |
| Sorry, bad picture, but the stick is in my left hand and you can make out the marks and the washer if you squint. |
First attempt was a little shaky in a slight breeze, so I attempted to steady it with my fingers.
Maybe what was needed was a more viscous pool, like mercury (Wikipedia says the Romans had it), or perhaps olive oil? I was too cheap to try that, so made a batch of Roman gravy, with flour, butter, water, a bit of milk and a dash of Mediterranean sea salt (because why wouldn't you?)
Even more effective was a couple of movable stops I attached to the bowl, just kissing the floating beam to keep it from moving in the wind.
If I had an assistant (a trusted slave) he could slide his finger down the yardstick until I told him he was lined up with my nail points, and one of us would record the rise (XVIII inches in this case).
 |
| The top of the blue tape on the stick is the "reference"- the same height as my nail points |
When we laid out my house, we used a fancy expensive laser version, but the principle was the same, and my brilliant engineer buddy assures me that accurate elevation surveying, including over long distances, is still done in roughly the old Roman way, one sight at a time, inch-worming over hill and dale.
Oh, and Bigfoot was back today, hopefully dining on some of my delicious gophers.
