Wis. Stat. § 4.003: Description of boundaries.

Wherever in this chapter territory is described by geographic boundaries, the following conventions are used:

(1) Each bound continues to the intersection with the bound next named, or to the intersection with a straight-line extension of such bound.

(2) If the bound is a street, it follows the center line of the street or the center line of the street extended.

(3) If the bound is a railroad right-of-way, it follows the center line of the railroad right-of-way.

(4) If the bound is a river or stream, it follows the center of the main channel of such river or stream.

(5) If the bound follows a municipal boundary, it coincides with such boundary.