The groma is composed of a supporting rod whose lower end consists of a point that was planted in the ground, or in a stone benchmark that the surveyor carried with him. Between the rod and the groma there was a movable arm, called the rostrum, which was free to rotate: the surveyor sighted on this to determine the desired orientation. On the rostrum was the groma itself, a wooden cross with iron fittings, from the extremity of which hung in two pairs four plumb-lines: the perpendicular of these on the ground determined the lines to be traced.