The groma was a simple sighting cross — two arms at right angles with plumb-bob cords — mounted on a staff, used with the chorobates level and the ten-foot decempeda measuring rod. Standing at a fixed point, the surveyor sighted along both arms to set out two perpendicular axes, the decumanus and the cardo, then repeated the operation to lay a field of squares. Applied to territory this produced centuriation: land divided into centuriae of 20 by 20 actus (about 710 m square, roughly 50 ha), each subdivided into holdings and registered on bronze or stone cadastral maps (formae). The same instrument set out the street grid and insulae of new colonies.
Centuriation made the survey grid an instrument of colonisation and land allocation, not just of drawing: it distributed veteran plots, fixed property boundaries and drainage lines, and tied the city plan to the surrounding field system. Its traces still govern road alignments and parcel patterns across the Po Valley, Tunisia and southern France. It is the direct ancestor of later cadastral grids from the Spanish Laws of the Indies to the US township-and-range survey.
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