Looking in Yard:
Yes. The police can look into your yard with binoculars, from airplanes, from a neighbor’s yard, and from just about anywhere. If an officer is lawfully in a place from which he can view your home or yard then it is considered to be within the “plain view” exception to the search warrant requirement.
Walking in Yard:
No. The police must have a proper search warrant to enter premises absent consent (or “exigent circumstances”). The term “exigent” – by its nature, means an emergency. Absent this, they cannot walk onto the yard of your premises. The law requires the police to obtain a search warrant from an impartial magistrate judge before they can enter your home (or yard).