A federal judge has ruled that police officers in Wisconsin did not violate the Fourth Amendment when they secretly installed cameras on private property without judicial approval.

The officers installed the cameras in an open field where they suspected the defendants, Manuel Mendoza and Marco Magana, were growing marijuana. The police eventually obtained a search warrant, but not until after some potentially incriminating images were captured by the cameras. The defendants have asked the judge to suppress all images collected prior to the issuance of the search warrant.

But in a Monday decision first reported by CNET, Judge William Griesbach rejected the request. Instead, he approved the ruling of a magistrate judge that the Fourth Amendment only protected the home and land directly outside of it (known as "curtilage"), not open fields far from any residence.

The Fourth Amendment protects "the right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures." The court ruled that under applicable Supreme Court precedents, "open fields, as distinguished from curtilage, are not 'effects' within the meaning of the Fourth Amendment."

The property in question was heavily wooded, with a locked gate and "no trespassing" signs to notify strangers that they were unwelcome. But the judges found that this did not establish the "reasonable expectation of privacy" required for Fourth Amendment protection. In their view, such a rule would mean that (in the words of a key 1984 Supreme Court precedent) "police officers would have to guess before every search whether landowners had erected fences sufficiently high, posted a sufficient number of warning signs, or located contraband in an area sufficiently secluded to establish a right of privacy."

"The Supreme Court has upheld the use of technology as a substitute for ordinary police surveillance," the judges held. Because it would be legal for the police to enter a private field to collect evidence, they concluded it must also be legal to install cameras there.

But this illustrates the absurdity of automatically allowing the police to automate any surveillance activities they're allowed to do manually. In arguments over warrantless GPS surveillance, for example, the government has insisted that installing a tracking device on a suspect's car is no different than having an officer manually follow the car, an activity that has always been permitted by the Fourth Amendment.

But the police only have the resources to tail a small number of suspects at any one time, and a suspect is likely to notice a car following him 24/7. So allowing the installation of GPS tracking devices dramatically increases the government's practical spying powers even if the formal law hasn't changed. And that, in turn, can diminish the privacy protections that are the whole point of the Fourth Amendment.

The same point applies to the installation of surveillance cameras. The police can't afford to put police officers in the bushes of every suspected marijuana grower. But they can afford to install surveillance cameras in every suspect's back yard. So allowing warrantless camera installation dramatically increases the police's surveillance powers, which ought to attract stricter judicial scrutiny.

Update: Our original story incorrectly suggested that Mendoza or Malaga owned the property in question. As the magistrate judge explained in a footnote: