Trespass to land is a tort that is committed when an individual or the object of an individual intentionally or negligently enters the land of another without a lawful excuse. In other words A will commit the tort of trespass to land if he crosses the boundary of B’s land,when he has no lawful justification or excuse for doing so. A will also commit the tort of trespass to land if he directly causes some object or matter to pass onto B’s land when he has no lawful justification or excuse for doing so.  In some jurisdictions, this rule may also apply to entry upon public land having restricted access.

In order to establish that A committed the tort of trespass to land, B does not have to prove that when A crossed the boundary he intended to trespass on B’s land; or that when A crossed the boundary, he knew or ought to have known that there was a risk he would trespass on B’s land.   Indeed A will have committed the tort of trespass to land even if, when he crossed the boundary of B’s land, he honestly believed that he was remaining on land where he was permitted to be. An honest mistake by A is not a defence to the tort of trespass to land.  
It must be noted that trespass is actionable per se, which means that it is actionable regardless of whether B suffers any consequential damage.  However, suppose that A at some time crossed the boundary of B’s land. A will only have committed the tort of trespass to land when he crossed the boundary of B’s land if he were in control of his body when he crossed the boundary.If A forces B unwillingly onto C’s land, C will not have action in trespass against B, because B’s actions were involuntary. C may instead claim against A. Furthermore, if B is deceived by A as to the ownership or boundaries of C’s land, A may be jointly liable with B for B’s trespass.  
For a trespass to be actionable, the defaulting party must voluntarily go to a specific location, but need not be aware that he entered the property of a particular person. In most jurisdictions, if a person were to accidentally enter onto private property, there would be no trespass, because the person did not intend any violation. However, in Australia, negligence may substitute the requirement for intent.   Moreover, B will only be able to sue someone for trespass on a particular piece of land if he was in possession of the land at the time of the alleged trespass. Possession here does not mean physical possession but actually legal possession. Where the land is held under a tenancy or lease,it is the tenant that will sue.
If a trespass is actionable and no action is taken within reasonable or prescribed time limits, the land owner may forever lose the right to seek a remedy, and may even forfeit certain property right. In some jurisdictions trespass while in possession of a firearm, which may include a low-power air weapon without ammunition, constitutes a more grave crime of armed trespass.    
If A has crossed the boundary of B’s land, there are many different ways in which A could establish that he had a lawful justification or excuse for crossing that boundary,some of these excuses include;
1. Licence ( consent) meaning that he had the landlord’s permission to enter into the land.
2. Necessity meaning that there was a case of immediate danger or emergency which neccesitated the entry into the other’s boundary.
A court may order payment of damages or grant an injunction restraining anyone from further entering into the land  as remedy for the tort.
Adedunmade Onibokun Esq.