The surveyor liked to link up his centuriated grid with a colony plan. Thus at Cosa the _groma_, for siting the allotments, could have been set up in the Porta Romana (the northeast gate), and at Alba the line of the Via Valeria inside the walls, if projected, would cut the lines of centuriation at right angles. The four sides of the _centuria_ were usually marked by roads, the inner subdivisions by narrower roads, trees, hedges, or drainage or irrigation ditches. Modern land-use often follows the line of the ancient: one stretch recently laid out and now in use at Sesto, west of Florence, deliberately follows the traces of Roman centuriation, restored by a classically trained engineer for modern man to admire. As with the grid inside a colony wall, the centuriated grid of allotments was laid out from a basic _cardo_ and _decumanus_. The Roman surveyors were balked by no natural barriers. Bradford cites one line of centuriation running as high as 1600 feet above sea level (though within the _centuriae_ the furrows might follow the contours) and another, in Dalmatia, continues from a peninsula across to the mainland, spanning an arm of the Adriatic Sea three miles wide. In north Italy, where the flatlands of the Po Valley made the survey easy, one can ride from Turin (Roman Augusta Taurinorum) to Trieste (Roman Tergeste), three hundred miles, through centuriated systems all the way. The same air photographs which revealed neolithic sites to Bradford in Apulia showed Roman centuriation, too, and subsequent digging turned up pottery of Gracchan date (about 133–122 B.C.). A particularly extensive stretch, outside of Italy, is found in Tunisia. It has been traced from the air 175 miles from Bizerta to Sfax, and southwestward from Cape Bon for 100 miles inland. It probably goes back to ambitious plans of Gaius Gracchus, about 122 B.C., to resettle Rome’s urban proletariat. [Illustration: FIG. 4.10 Paestum: Roman grid of streets (air-photograph). (Italian Ministry of Aeronautics)]