By derivation, a _colonia_ is a place where men till the soil. Colonists were assigned centuriated allotments. Since traces of centuriation have been found both at Alba Fucens and at Cosa (Figs. 4.8 and 4.9), as well as at nearly fifty other certain and half as many possible sites in Italy, this seems an appropriate place to discuss the subject. Wherever colonies were planted, wherever land was captured, confiscated, redistributed to the poor or to veterans, the surveyor with his _groma_, or plane-table, was on hand. Air photography is a great help in revealing traces of the Roman surveyor at work, for modern land-use has often overlaid the ancient traces, leaving ancient crop-marks as the only clue. The standard surveyor’s unit was the _centuria_ of 200 _iugera_ (the _iugerum_, five-eighths of an acre, being the area an ox could plow in a day), and a side of twenty _actus_ (776 yards), its corners marked by boundary stones, some of which survive. There has been too little digging to confirm the results of air reconnaissance, but it seems clear that some centuriation goes back to the late third century B.C. Dr. Ferdinando Castagnoli, the Italian expert, is inclined to date that of Alba and Cosa at least this early, as well as large stretches in the fertile Campanian plain northwest of Naples. [Illustration: FIG. 4.8 Alba Fucens, centuriation. (F. Castagnoli, _Bull. Mus. Civiltà Rom._ 18 [1954–1955], p. 5)] [Illustration: FIG. 4.9 Cosa, centuriation. (F. Castagnoli, _loc. cit._, p. 6)]