[podcast]https://edwardwillett.com/wp-content/uploads/2009/04/surveying-technology.mp3[/podcast]
I’m working on a history of the Saskatchewan Land Surveyor’s Association—and, as with everything I work on, learning stuff I never knew before.
In this case, stuff about the technology of surveying.
The ancient Romans did pretty well using just three simple instruments: the groma, the chorobate, and measuring rods.
The groma consisted of crossed arms resting on a bracket attached to a vertical staff. From each of the four arms hung a plumb bob. The groma could be used to survey straight lines, right angles and rectangles.
The chorobate was a ten-foot long wooden trough with a groove running down its middle. Water was poured into the groove; if it pooled instead of running out either end, then whatever the chorobate was placed on was level.
The Romans’ measuring rods were generally 10 Roman feet long (a Roman foot was a bit shorter than a modern foot). Long distances were measured using stretched-out wax-covered cords.
The next big improvement in surveying, a means of measuring angles between landmarks, was the astrolabe. Developed around 400 A.D. and perfected in the Islamic world, the astrolabe came to Europe in about the 12th century.
An astrolabe shows how the sky looks at a specific place at a given time. It can be used to find the time, figure out when sunrise and sunset will fall, or locate celestial objects. At sea, a simpler device called a mariner’s astrolabe was used: it was a ring marked in degrees for measuring the angle above the horizon of objects in the sky. The mariner sighted the objects along a rule called an alidade.
Hard on the heels of the astrolabe’s arrival in Europe came the compass, and in 1533 Gemma Frisius combined the two into the Epipedometron—which most surveyors didn’t use because it was too complicated. Instead, they used the plane table, nothing more than a smooth table that could be precisely leveled (it was usually on a tripod).
Surveyors would place an alidade on the plane table and sight along it to a particular object, then draw a line to indicate the bearing. If you have two such tables a known distance apart, and take a bearing from each to a third object, you can precisely locate that third object on a map: it’s where the two bearing lines cross. This is known as triangulation.
Once you’ve created one triangle of three known points, you can use one side of it to create another triangle, and so on, until you’ve surveyed as large an area as you wish.
All of these various methods came together in what most of us today think of as the quintessential surveying tool, the theodolite, a single device that allows a surveyor to both take bearings and measure elevations. Early theodolites used alidades. The 18th-century addition of telescopes greatly increased their accuracy, as did the addition of bubble levels. The transit is a 19th-century improvement in which the telescope can “transit,” or flop over, allowing the surveyor to sight back the opposite way.
Distances were measured by special chains or, later, metal tapes, but then the Second World War gave us electronic distance measuring (EDM) equipment in the form of radar, which could precisely locate an object by bouncing radio waves off of it.
The geodimeter accomplishes the same thing for surveyors by using light. Most are short-range, using infrared over a couple of kilometers, but use lasers and the effective range is immense: NASA bounced lasers off reflectors the Apollo astronauts left behind to precisely measure the distance to the moon, which averages 384,467 kilometres.
The transit and EDM combined with the computer revolution in the 1970s to produce the total station. It uses an electronic transit to measure angles and elevations, infrared EDM to measure distances, and a computer to automatically perform calculations and record everything for later downloading.
And then along came the Global Positioning Satellite system, now in such wide use that you can get a receiver in your cell phone that allows you to fix your position using signals from orbiting satellites. According to the official GPS website, “Today it is possible for a single surveyor to accomplish in one day what used to take weeks with an entire team.”
In practice, of course, a variety of technologies may be used on any given surveying job. Next time you see a surveyor in the field, ask him to show you his equipment.
Um…on second thought, you might want to rephrase that question.
