Wis. Stat. § 898.15: Jail liberties.

A space of ground in a square, the center of each of whose sides shall be one mile distant from the jail, is declared as the liberties of the jail of each county of the state. The sheriff of each county, where such liberties have not been heretofore so designated, shall, at the expense of the county, designate by visible and permanent marks the extent and limits of such liberties.