groma
Quick Summary
The groma was a surveying instrument used by the ancient Romans to establish right angles. It consisted of a horizontal wooden cross, pivoted at the center and supported from above. A plumb bob hung from each of the four arms. By sighting along pairs of plumb bob cords, surveyors could accurately establish a right angle. The groma could be adjusted for precision by turning it approximately 90 degrees and correcting any error by shifting one of the cords. While the Greeks may have originated its use, Roman surveyors made it a standard tool in their practice.1
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