To your original question..........is the trespasser on your land by accident or knowingly trespassing? I am assuming he or they may be trespassing knowing full well its your land.

If by accident, let them know they are on the wrong land unless they are leaving trash, etc. It can happen and has happened to me before, because of miscommunication. If they have previously been told they do not have permission to be there then by all means call the authorities. The reason I say this.....a plat book could be wrong or perhaps another landowner gave them wrong information.

I am a co-owner of land in Indiana and my siblings and I always at least tell the guys they do not have permission.