In my area if the land is not posted and you have warned them and they still trespass, then they can be charged. Also you have to have it posted at the county/town hall, signs alone does not mean it is legally "posted."
The law called me Saturday and told me I don't have to appear in court, he paid his ticket off. Also he got his truck back. (The officer said the whole time he was in the magisrates(sp?) office, his wife was raising hell.)