The Roman Army A to Z: groma

groma (f. pl. gromae)

1. A surveying instrument consisting of a staff supporting a crosspiece with four plumb-bobs. 2. The point from which a camp was laid out, named after 1. 3. The name of a quadrifrons arch (or tetrapylon) sometimes erected over 2., at the junction of the via praetoria and via principalis, and at the entrance to the principia. DMC 12; AE 1974, 723. [Johnson 1983]