Ground on which each side has liberty of movement is open ground.
This is only a makeshift translation of 交, which according to
Ts‘ao Kung stands for 交錯 “ground covered with a network of
roads,” like a chess-board. Another interpretation, suggested by
Ho Shih, is 交通 “ground on which intercommunication is easy.” In
either case, it must evidently be 平原 “flat country,” and
therefore 不可杜絶 “cannot be blocked.” Cf. 通形, X. § 2.