Legally:

The act of “trespassing” is often conceptualized as one person setting foot on another person’s land without their consent. “Get off my property, you’re TRESPASSING!” is a common phrase landowners employ when faced with uninvited guests.

At law, the term “trespass” encompasses much more than individuals creeping onto their neighbour’s property without permission. In fact, a landowner’s right to be free from intrusions (be it people or objects) extends to both the air above and the ground below their land. This legal principle stems from the latin maxim, cuius est solum eius est usque ad coelum et ad inferos, which roughly translated means: for whoever owns the soil, it is theirs up to heaven and down to hell.

Lamentably:  An excerpt from Henry David Thoreau’s “Walking” [1862]

At present, in this vicinity, the best part of the land is not private property; the landscape is not owned, and the walker enjoys comparative freedom. But possibly the day will come when it will be partitioned off into so-called pleasure-grounds, in which a few will take a narrow and exclusive pleasure only,—when fences shall be multiplied, and man-traps and other engines invented to confine men to the public road, and walking over the surface of God’s earth shall be construed to mean trespassing on some gentleman’s grounds. To enjoy a thing exclusively is commonly to exclude yourself from the true enjoyment of it. Let us improve our opportunities, then, before the evil days come.

Sources:

http://www.lawnow.org/no-trespassers-allowed/

http://www.bartleby.com/28/15.html

Picture Source:

http://www.fotolibra.com/gallery/364382/no-public-right-of-way-sign/like/