A.R.S. § 11-121: Action to determine boundary

When a dispute arises between two counties respecting the location of the boundary line between such counties, or when the boundary line between two counties is so indefinite as to make it impossible to determine where the county line is, and a portion of territory by reason of the indefinite description is claimed by two counties, either county may commence an action against the other in the supreme court for the purpose of having the boundary line determined.