The groma was an ancient survey instrument used often by the Romans. Its origins may go back to Mesopotamia as early as the 4th century BCE. It was a complicated piece of equipment consisting of a vertical staff with right-angle crosspieces attached horizontally. Each crosspiece had its own plumb bob. The groma was limited to surveying straight lines and perpendiculars, but it allowed for simple measuring of rectangular areas (Illustration by Dan Patterson).
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