The First principal meridian originates at the junction of the Ohio and Great Miami Rivers. It then runs 175 miles northward, along the Ohio-Indiana state border, before terminating at the northeastern corner of Indiana. First surveyed by William Harris in 1817, it roughly approximates to the meridian of longitude 84°48’50”W from Greenwich.
Congress Lands West of the Miami River use the First principal meridian as the basis for numbering ranges eastward into Ohio. Additionally, there is a triangular sliver of land (highlighted above) stretching westward into modern day Indiana, south of the Greenville Treaty Line, that was surveyed as part of Congress Lands West of the Miami River. Ranges in this sliver of land are numbered westward into Indiana from the First Principal Meridian. Cadastral surveys done of Congress Lands West of the Miami River use the Ohio River as a crude base line from which to number townships northward. This early method of numbering townships from natural boundaries such as the Ohio River mirrors the method used in the Ohio River Base surveys, however, this method created some irregularities in the numbering of townships as can be seen by comparing R2W and R3W in the viewer where adjacent townships do not share the same number as is customary in modern cadastral surveys of public lands. As the Ohio River meanders it creates a "stairstep" pattern in the numbering of townships which can cause confusion when compared to the neat grids laid out in states to the west of Ohio.
Townships were subdivided into sections which were numbered as per the Land Act of 1796.