Hester v. United States first introduced the doctrine that the Fourth Amendment protection does not extend to open fields. Governmental intrusion and information collection upon open fields do not constitute searches or seizures under the Fourth Amendment. Expectation of privacy in an open field is not considered as reasonable or legitimate, even if there are fences or “no trespassing” signs around the field. Open fields may include any unoccupied or undeveloped area outside the curtilage.
Some states reject open field doctrine and confirm protection under state law of privately owned open lands from warrantless searches and seizures, where a reasonable person should know that the occupant intends to exclude public and precautions have been taken. For instance, in State v. Dixon, the Supreme Court of Oregon held that Article I, Section 9 of the Oregon Constitution, i.e. the search and seizure provision, applies to privately owned lands outside the curtilage of a residence.
Hester v. United States: http://www.law.cornell.edu/supremecourt/text/265/57