According to the website for the Whitewater River Foundation: In October 1798, Israel Ludlow, Deputy Surveyor and a founder of Cincinnati, was appointed by President George Washington to survey this section of the Northwest Territory. He ran a line north from the middle of the mouth of the Great Miami River, as a meridian from which to lay off the government townships and sections. The lower part of the Ohio-Indiana line was thus first laid out for use as a standard meridian for land surveys and not for a state boundary.
There is apparently a stone marker as part of the survey. I’m going to have to go check this out!