Could well be and should be.
Height and width specifications from The NC Wildlife Resources Commission:
Why Purple Paint?
Landowners in North Carolina who want to post their lands can have difficulty keeping posted signs erected and intact. Using paint marks, as an alternative or in addition to signage, is a convenient and effective means of marking lands as posted, and requires less frequent maintenance and cost, since paint marks are more difficult to vandalize than signs.
Many states throughout the U.S. currently allow the use of paint marks to denote land posting. Landowners in N.C. now may use signs, purple paint marks or both to post their properties.
Each paint mark must be a vertical line of at least eight inches in length, and the bottom of the mark shall be no less than three feet or more than five feet from the base of the tree or post. For more information, including an illustration, on how to post property under the new law, see the Landowner Protection Act document on our website.