Free: Contests & Raffles.
A lot of their info comes from county records so....
No, the lines shown on your onx map are not necessarily where the actual property lines are. If the property has been surveyed and the survey filed with the county as required by law, then chances are the onx map WILL get you within 10 feet of the actual line. But if there's never been a survey done then those lines were just drawn in by the guy in the assessor's office and could easily be a couple hundred feet off, or even more.
Historic property boundaries vs. current GPS property boundaries are typically off.My neighbor sold a portion of his property so they had it surveyed with new technology.The boundaries shifted about 6 feet to the north, which doesn't align with the obvious "historic" boundary.You can make historical boundaries real boundaries thru a legal process.
It definitely does not upgrade every year. I don't know the rate of when they update but I have bought some property in Okanogan County four years ago and it still has the old owners info on it.
Gps is not exact. Can be perfect can be off by 200 ft. Uncle sam incorporates errors on purpose as antiguidance precautions