The answer to your question may take several hours of legal research. A few court hearings may be
required leading up to a trial. Documents will have to be exchanged between the sides.
The tape measure evidence will not hold up in court, though, and a pro is needed for the survey, as you know.
Because this matter is so important you should really get a lawyer.
You might find my Legal Guide helpful "How to Choose A Lawyer For You"
You might find my Legal Guide helpful " What Do I Tell My Lawyer"
No one can know what the record is in the case because online we cannot see your documents. You need a lawyer. Check with a lawyer in your locale to discuss more of the details.
Good luck to you.
NOTE: This answer is made available by the lawyer for educational purposes only. By using or participating in this site you understand that there is no attorney client privilege between you and the attorney responding. This site should not be used as a substitute for competent legal advice from a licensed professional attorney with whom you have established an attorney client relationship and all the privileges that relationship provides. The law changes frequently and varies from jurisdiction to jurisdiction. The information and materials provided are general in nature, and may not apply to a specific factual or legal circumstance described in the question.
You should consult with a real estate attorney licensed to practice law in Kansas. In California, this would be a straight-forward case because the "agreed boundary" doctrine does not apply when the true boundary lines can be determined by a boundary survey. An expert in real estate law should be able to tell you off the top of their head what will be involved in litigating your case.