Back in the day when life was simple, and the only objects in the air were birds, arrows, and the occasional cannon ball, English law took a simple approach to a landowner's rights to the air above his property. He was the owner of the property "all the way to heaven and all the way to hell." (2 Blackstone Commentaries Sec. 18.) This "heaven to hell" doctrine was adopted by most American states, which gave landowners rights to everything below and above their property, period. At least one state, Montana, retains this view, though its validity is questionable: "The owner…has the right to the surface and to everything permanently situated beneath or above it." (Mt. Code Sec. 70-16-101.)
Literally applied, the English rule could result in a landowner refusing to allow an airplane to enter its airspace (as long as the plane stayed below heaven). The absurdity of this result was rejected in a 1946 U.S. Supreme Court case, U.S. v. Causby, 328 U.S. 256 (1946). The Causbys were chicken farmers whose farm was in the glide path of military aircraft approaching a nearby airport. The planes came within eighty-three feet of the ground, and the terrified the chickens stopped laying; many died while running from the noise. The Causbys were forced to abandon their business, and sued the government for trespassing and then essentially taking their property – their airspace – without compensating them.
Had the court been willing to apply the "heaven to hell" theory, the Causbys would have won easily. Actually, they did win, but made some important law in the process. The Court ruled that the landowner "owns at least as much of the space above the ground as he can occupy or use in connection with the land." (U. S. v. Causby at p. 264.) Everything above was navigable air space, available to the public. The Court didn't announce a precise altitude after which landowners lost their rights. Instead, they offered two general guidelines that would be applied in each case:
Applying these guidelines to the Causby situation, the Court ruled that the airplanes, at 83 feet above the ground, were interfering with airspace that belonged to the farmers. They were trespassing and depriving the owners of the use of their land. That constituted a "takings" under the Fifth Amendment, and the government was ordered to pay damages to the Causbys.
You might be tempted to conclude that a drone that flies below the navigable airspace but above private property is trespassing. But it's not so simple, as you will see.
Federal law (The Air Commerce Act) gives the government exclusive control over "navigable airspace." The FAA defines and regulates navigable airspace, through which the public has a right of transit. Today, navigable airspace is defined as the air space above the lowest altitude at which airplane flight is authorized (500 to 1,000 feet, depending on the area). It also includes air space needed for takeoff and landing.
State laws, too, have a say in how much air space a landowner owns and can control. For example, an owner generally has the right to remove tree branches from a neighbor's tree that overhang the property line (the overhang is considered a trespass). Similarly, an owner may consider a hunter to have trespassed if the hunter fired a shot that traversed the owner's property. State and local governments have enacted building height restrictions and laws prohibiting view obstructions. And some states have made drone flights above private property a trespass, period. But when it comes to laws regarding navigable airspace, the FAA has the final say. In fact, the FAA has taken the position that it has the power to regulate aircraft, including drones, at any altitude, including that below flight levels.
Reading the explanation above, you can probably see the collision course we're on: Landowners have the right to control at least some of the usable space above their land, yet the FAA claims the power to regulate drone use below, as well as within, navigable air space. Add to that a state's law that makes drone flight above private property a trespass, and you have at least the following conundrums:
As you can see, there are no easy answers. At the very least, a drone that flies substantially below the flight level, and that does so without FAA approval (that is, recreationally), is ripe for a lawsuit, particularly if local or state law clearly limit drone usage.