The isolation and distribution of measurement errors is one of the core functions of a land surveyor.
If I were building a new GIS for tax mapping I would make no assumptions and simply deal with error by calculating the area of the overlapping parcel-polygons.
You would still need to make a best fit layer as the user default, but having a nested layer showing the overlap or gap should help to shield you from the various liabilities associated with gross misrepresentation of real property holdings. It would also make people aware that's time to get it surveyed which would provide you with an accurate plat.
I'm just stating how coordinate systems work, we presume all land surveys are done on the horizontal, which is how they work out here where I am, sounds like older ones in WV may have been done on the slope.
If he is doing what is usually done with most GIS maps, then he is putting it down on the state coordinate system, it's good he is asking questions, but my advice is to hire a local surveyor to help out putting it together.
I worked up a bid to do the local county GIS back in about 1990, they went with a group who put in a bid to do it and be done in 6 weeks.............a jaw dropping proposal if there ever was one.
Years and years later they were still fighting it. Finally they went out and hired a guy who is a surveyor and experienced with doing GIS and finally, finally they have a nice product, still not very close to "accurate" but so much better.
So I'm very jaded when it comes to problems with tax mappers. Get a surveyor in there, it will cost some, but pay in the long run.
