This is precisely the way the agreement is supposed to work. SIMPLE and INEXPENSIVE. When you stop to think about it, EVERY boundary ever established between two properties is established by some form of agreement. The original grantor/grantee agreed where the boundary was to be located (if they even discussed location) or they agreed to locate it at some later point in time. When that point in time arises, you may not even be dealing with the same parties to the original agreement, therefore, the boundary is unknown or uncertain.
Even when they hire a professional surveyor to discover the location of the line, there is nothing binding about the survey results. After the surveyor walks away from the uncalled-for monuments that were just set, the owners begin their improvements and establish their boundary through some form of agreement. We like it when they call us, and we also like it when no one challenges our survey and exposes the errors we just monumented. Doesn't matter any more anyway, because the landowners have agreed on the location and established it through the assertive act of constructing improvements. The boundary is fixed as located. We just hope we didn't blunder too badly or we may be paying damages for someone's property rights that were affected by our survey.
When they don't hire a surveyor and choose to "go it on their own," their agreement to establish the boundary is binding. A subsequent (future) survey of the property should expose the evidence of the agreement and document it.
I found one recorded boundary agreement in an older area of town where surveys were historically messy that identified the two adjoining owners and stated that the agreed boundary was the fence line running in a generally north-south direction located 28 feet westerly of the northwest corner of the foundation of the residence. Of course there was an addition put on the home, so I had to take a close look at the foundation lines to see where the old corner was when the agreement was written. Fence was still there. Guess where the boundary was? Right where they agreed it was.
JBS
Consider the case of a large timber company. They own the southeast one-quarter of the northeast one-quarter of an interior Section that is reasonably close to 80 chains square. The country is broken, brushy and steep. They own no other lands in this particular Section. Their Surveyor (100+ years ago) starts at the east quarter section corner and runs west (by compass and chain) 20 chains and sets a post at the center east 1/16th corner; then he runs north 20 chains and sets a post at the northeast 1/16th corner, then he runs east 20 chains and sets a post at the north 1/16th corner on the east line of the section, then he runs south 20 chains to the point of beginning. If the timber company owns the lands to the east he might check into the northeast section corner.
This company doesn't want to run miles of line in lands they don't own just to subdivide a section they mostly don't own. Even if they did so their accuracy would probably not be very good. A reasonable person thinks a quarter-quarter section is 40 chains square. The other small owners in the section are glad the timber company set those posts because it saved them that expense. Now they have cost-free established corners to run from for their needs. Everyone is happy. There is peace in the land. Everyone is in agreement at least until the technocrats arrive.
The owners cut up their lands by Deeds and use those posts to measure out the new boundaries. If it says start at a corner and run east 800' the fence corner might be anywhere from 790' to 810' east of post representing that corner. No way did they have in mind to start at some imaginary point and run east 800.0000000' to some other imaginary post. It is obvious to anyone with a modicum of common sense what they actually did.
Granted if the owner of the northeast of the northeast has need to know his boundary he can accept the Timber Company line or have his own survey made. If he accepts the Timber Company line that doesn't affect the owners to the west but it does establish the north-south location of the east-west centerline of the northeast quarter of the section. Sometimes things are done piecemeal making a mess; that is reality.
Suppose the owner of the northeast of the northeast accepts the timber company line. The line is well marked and both parties obviously occupy up to it. Some Decades later the northeast quarter is found to be only 35 chains in north-south dimension. This does not now move the 1/16 line south to favor the northerly owner. The line is already agreed upon. Both owners have clear title up to the established line by the same tenure they hold the main body of their lands (presumably a Grant Deed). The only requirement to establish the validity of the agreement is that they did not know where the boundary was located on the ground (a dashed line on the Plat does not constitute certainty, the line has to be marked upon the ground).