Measuring like a Viking by Coopting Roman Tools
Groma
Originally uploaded by Ragnvaeig. One of the excavators at Fyrkat suggested a very Roman method of surveying for Viking circular fortresses, down to the use of the groma--an example of which I built out of items from stash and my neighbourhood Big Box Hardware Store so that I could show my Viking Civil Engineering students how to use it.
The upright is a thick round dowel, and I cut and lapped the crossed arms together from a single square dowel. The support bracket is a hanging plant bracket through which I drilled a hole with a metal bit, and the swivel which connects all these pieces (and suspends the central plumb bob) is an eye bolt and a couple nuts. The plumb bobs I made out of linen cords and glass beads, though I could also have gotten much heavier plumb bobs at the hardware store. There are pages online dedicated to the design and construction of very proper gromas, with metal uprights and a spike to drive them into the ground, but this is the "you, too, can build one of these at home!" model for encouraging students to try it themselves.
You use the groma first by offsetting the crossed arms from the supporting bracket: if you leave them the way they are pictured here, the upright will block your view of at least one of the plumb bobs. You line up the three plumb bobs of one arm to help you draw your first straight line, then you sight along the three plumb bobs of the other arm to survey a line orthogonal (at 90º) to the first.
We used this in my Viking Civil Engineering class for drawing the roadbeds for the arterial lanes in the 1:30 circular fortress we laid out. It worked very well, and I'm quite pleased with how well it works for about US$15 worth of materials. We were proper agrimensores!
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