Roman Groma


Figure 1. Groma center cross plumb line over survey marker. Figure 2. Groma plumb lines aligned for sighting straight line.

Roman surveyors (Agrimensores) used a groma for laying out roads, buildings and aqueducts.  Figure 1 shows a model of a groma.  A groma consisted of a metal drive point attached to the bottom of a pole. A swivel arm extended from the top of the pole.  Attached to the end of the swivel arm was a cross made with  two boards that could be rotated in a circle.  Extending down from the center point of the cross was a plumb line with a plumb bob.  The groma was installed in the ground such that the plumb line extending down from the center point of the rotating cross was directly over the survey marker on the ground. Attached to the ends of the boards that formed the cross were plumb lines.  To survey a straight line from the survey marker, the Agrimensore would position the groma over the survey mark then rotate the cross to align two of the plumb lines at the end of the wooden cross with the center plumb line as shown in Figure 2.  He would  then sight along the three aligned plumb lines to survey poles moved into alignment by assistants (likely slaves).  The survey line would be extended by moving  the groma to one of aligned pole positions and repeating the procedure. 

Want to try a Roman groma to survey for an aqueduct in virtual space?   You will need the free adobe shockwave player.   Then go to Groma Simulation and follow the directions.


Want to read more about gromas? Visit these references:


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