you don't "have to" but you have to "give notice"

State Law
Penal Code § 30.05. Criminal Trespass.

(a) A person commits an offense if he enters or remains on or in property, including an aircraft or other vehicle, of another without effective consent or he enters or remains in a building of another without effective consent and he:
(1) had notice that the entry was forbidden; or
(2) received notice to depart but failed to do so.
(b) For purposes of this section:
(1) “Entry” means the intrusion of the entire body.
(2) “Notice” means:
(A) oral or written communication by the owner or someone with apparent authority to act for the owner;
(B) fencing or other enclosure obviously designed to exclude intruders or to contain livestock;
(C) a sign or signs posted on the property or at the entrance to the building, reasonably likely to come to the attention of intruders, indicating that entry is forbidden;
(D) the placement of identifying purple paint marks on trees or posts on the property, provided that the marks are:
(i) vertical lines of not less than eight inches in length and not less than one inch in width;
(ii) placed so that the bottom of the mark is not less than three feet from the ground or more than five feet from the ground; and
(iii) placed at locations that are readily visible to any person approaching the property and no more than:
(a) 100 feet apart on forest land; or
(b) 1,000 feet apart on land other than forest land; or
(E) the visible presence on the property of a crop grown for human consumption that is under cultivation, in the process of being harvested, or marketable if harvested at the time of entry.
(3) “Shelter center” has the meaning assigned by Section 51.002(1), Human Resources Code.
(4) “Forest land” means land on which the trees are potentially valuable for timber products.
(5) “Agricultural land” has the meaning assigned by Section 75.001, Civil Practice and Remedies Code.
(6) “Superfund site” means a facility that:
(A) is on the National Priorities List established under Section 105 of the federal Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (42 U.S.C. Section 9605); or
(B) is listed on the state registry established under Section 361.181, Health and Safety Code.
(c) It is a defense to prosecution under this section that the actor at the time of the offense was a fire fighter or emergency medical services personnel, as that term is defined by Section 773.003, Health and Safety Code, acting in the lawful discharge of an official duty under exigent circumstances.
(d) An offense under Subsection (e) is a Class C misdemeanor unless it is committed in a habitation or unless the actor carries a deadly weapon on or about the actor’s person during the commission of the offense, in which event it is a Class A misdemeanor. An offense under Subsection (a) is a Class B misdemeanor, except that the offense is a Class A misdemeanor if:
(1) the offense is committed:
(A) in a habitation or a shelter center; or
(B) on a Superfund site; or
(2) the actor carries a deadly weapon on or about his person during the commission of the offense.
(e) A person commits an offense if without express consent or if without authorization provided by any law, whether in writing or other form, the person:
(1) enters or remains on agricultural land of another;
(2) is on the agricultural land and within 100 feet of the boundary of the land when apprehended; and
(3) had notice that the entry was forbidden or received notice to depart but failed to do so.
(f) It is a defense to prosecution under this section that:
(1) the basis on which entry on the property or land or in the building was forbidden is that entry with a handgun was forbidden; and
(2) the person was carrying a concealed handgun and a license issued under Subchapter H, Chapter 411, Government Code, to carry a concealed handgun of the same category the person was carrying

This post was edited on 10/13/10 at 4:08 pm