owner disputing property line

I think that what you are thinking about here is something known as "adverse possession", sometimes referred to as "squatter's rights".  Under this doctrine, if certain requirements are met, one can legally take another's property without paying for it.  Unfortunately I don't think that you have adversely possessed the land in question for a long enough period.  Although I don't know if someone made use of that land prior to your use; that would make a difference.  Below are the statutory requirements for adverse possession in Missouri.

1.  Actual - You actually acted in the manner of an owner of the property

2.  Open & Notorious - You engage in acts of possession consistent with the property at issue in a manner which was capable of being seen. (This does not mean that you must have been observed in your acts of ownership but, had the actual owner or members of the public been in a position to see you, your acts must have been observable). You need not use the property in a manner that exceeds that which would be expected of the actual owner - that is, it may be possible to claim adverse possession of a vacation property on the basis of use only during the vacation season, or to claim adverse possession of a vacant parcel of land by engaging in typical acts of maintenance for the parcel.

3.  Exclusive - The adverse possessor does not occupy the land concurrent with the true owner or share possession in common with the public. One does not have to exclude others from the land in order to claim "exclusive" use, but during the statutory period the person claiming title by adverse possession must have been the only person to treat the land in the manner of an owner.

4.  Hostile- Hostility exists where a person possesses the land of another intending to hold to a particular recognizable boundary regardless of the true boundary line. That is, possession is "hostile" to the title owner's interest in the property. If possession was not hostile, it may still be possible to advance a claim of ownership under a theory of "acquiescence". You cannot claim "adverse possession" if you are engaged in the permissive use of somebody else's land.

5.   Continuous & Uninterrupted - All elements of adverse possession must be met at all times through the statutory period, 10 years, in order for a claim to be successful. It may be possible to claim adverse possession even if there is a transfer of ownership through the principle of "tacking" - for example, a former owner's four years of adverse possession can be "tacked" to the present owner's six years, for a cumulative ten years of adverse possession.