Most of the property line maps I've seen aren't terribly accurate. You need an actual survey, and that survey will include a deed search which will give the original boundaries along with any recorded agreements (Right-of-way, easement, etc.). Every map I've seen (except our actual survey map) has our property lines off by as much as 50 feet. Over a (nearly) half-mile property line, this is enough to short us 3 acres.