Disputes
These escalate faster than almost any other property problem, and they escalate mostly because of how they get handled rather than how far apart the lines are.
Not a plat, not a tape measure from the pin you think you found, not the county assessor's parcel map. A boundary survey by a licensed Missouri surveyor.
Assessor mapping in particular gets misused constantly. It exists for taxation, it is compiled from deeds at a scale that is not survey grade, and it is not a determination of where your boundary runs. People bring printouts of it to arguments and it settles nothing.
A survey gives you a defensible professional opinion based on records, monuments and boundary law. Sometimes it tells you the neighbor was right, which is worth knowing before you spend money on anything else.
This surprises people, and it is the most important thing on this page.
A survey establishes where the deed line runs. It does not automatically determine who owns the ground. Long-standing occupation can, in some circumstances, transfer rights regardless of what the deed says. Doctrines like adverse possession, acquiescence and practical location all exist for situations where a line has been treated a particular way for a long time.
Missouri has its own requirements and time periods for these, and whether they apply to your situation is a legal question rather than a surveying one. The practical point is this: a survey showing the fence is two feet onto your property does not automatically mean you get those two feet back. If the fence has been there for decades and both families treated it as the line, the outcome may be different.
This is where you need an attorney, not a surveyor. The survey is evidence. What it entitles you to is a separate question and a real estate attorney is the person to answer it.
What it does is convert an argument about opinions into a discussion about evidence, which is usually enough.