A boundary is fundamentally defined as the limits of space one is willing and able to defend from incursions and claims by others. Nations do this by force of arms. Failure to defend means one may lose that which is not defended.
Individuals in a country like the U.S. are expected to defend their property boundaries by enjoyment of the property and appeal to the courts when necessary instead of arms.
Similarly, failure to defend means one may lose that which is not defended.
There are, of course, many techniques and clever bureaucratic rules intended to administer and arbitrate this, but the fundamentals are very basic.
DEPENDS WHAT TYPE OF BOUNDARY IS.
BUT NORMALY ITS BY REGISTRATION FRAMES.
The range of my missiles.
It is the limits of real property described and conveyed through title documents and supported by physical evidence located by recovery or placement on the ground that can be retraced using historic or modern methods.
There are boundaries of record and boundaries on the ground. There are boundaries that are defined by monuments of record whether they are natural or artificial, there are boundaries that are defined by monuments that are not of record and there are recorded boundaries that have never been transcribed to the ground. there are boundary conflicts. Most boundaries can be quantified by going through the hierarchy of controlling evidence. But in the end, all boundaries are defined by the property owners. Whether amicably or through the court system. Not by the surveyor. Most clients understand the surveyor's rule when properly explained. But some clients just want to insist that surveyors are the almighty property line delineator. I make it a point to make sure all clients that I work with understands and agrees to my role before I agree to do any boundary retracement work with them. My two cents. Just be careful out there.
The best evidence according to the hierarchy of evidence....unless you're dealing with adverse possession.