From “Measuring America – How the United States was Shaped by the Greatest Land Sale in History” by Andro Linklater:

The best way to appreciate what Mansfield did is to drive west from Dayton, Ohio, on Interstate 70 and just across the Indiana state border to swing north on State Route 277. Beyond the wide expanse of the interstate, a beautiful rolling landscape can be glimpsed between predatory trucks and glaring billboards, but once on the narrow strip of 277, you become part of that country. The road acts as boundary to fields of wheat and soya, lawns with picket fences back onto it, and it is shaded by stands of oak and maple and walnut. The geological tsunami of the Alleghenies has virtually subsided here, leaving only ripples of rock beneath the surface. Sometimes blacktop, sometimes gray gravel, 277 runs straight as a surveyor's rule over the ripples, giving a ride as exhilarating as a powerboat at sea. In the troughs, all you can see are the nearest red-painted barns and green John Deere harvesters, but from the peaks the distant spires of churches and silver grain towers appear above the surging land.

However, Route 277 has another claim to distinction. Exactly one mile to the east, Indiana's border with Ohio runs parallel to it, a north-south line that Jared Mansfield designated as the First Principal Meridian. West of that line he was to establish a survey whose squares were so immaculate that their pattern would be compared to graph paper, checkerboards, and plaid. In other words, Ohio, with its different and indifferent surveys, was the proving ground for the system, but 277 marks the first line inside Mansfield's monumental gridiron.

Its regularity grew from the initial point formed at the meeting of the principal meridian - a carefully surveyed north-south line - and an eastwest baseline crossing it exactly at right angles. The squares were numbered outward from that zero point: Running east-west they were called ranges, and north-south they were townships; thus the first square west of a meridian and north of a baseline would be titled Range 1 West, Township 1 North.

Mansfield himself personally surveyed the Second Principal Meridian, whose initial point can still be found a few miles south of Paoli, Indiana, and others were run as new areas of land were pried from the Native Americans and put on the market. Instead of Rufus Putnam's independent survey districts, the different areas could be connected by extending a baseline or a meridian. The most spectacular example was the Fifth Principal Meridian, whose initial point was in Arkansas, near the present town of Blakton, 26 miles west of the Mississippi, but which was extended so far north that it eventually controlled other land surveys in Arkansas, Missouri, Iowa, South Dakota, North Dakota, and most of Minnesota, ending with Township 164 North, on the Canadian border.

Mansfield's method also provided a solution to the problem of converging meridians. The curvature of the earth brings lines of longitude gradually together as they run toward the pole, so that in most of the United States the northern end of a township is 30 to 40 feet narrower than the southern. In Alaska the flattening of the earth means that the lines close by more than 100 feet. (Some of Israel Ludlow's township meridians in Ohio converged by more than 300 feet, but that was not entirely the earth's fault.) As townships were stacked on top of each other, they grew narrower, and after four or five, the most northerly might be over 60 yards narrower than it was supposed to be. Consequently, Mansfield's successor, Edward Tiffin, decreed that a fresh start should be made at that point, with new meridians exactly 6 miles apart marked off on the baseline. The jog created by these "correction lines' where the old north-south line abruptly stopped and a new one began 50 or 60 yards farther east or west, became a feature of the grid, and because back roads tend to follow surveyors' lines, they present an interesting driving hazard today. After miles of straight gravel or blacktop, the sudden appearance of a correction line catches most drivers by surprise, and frantic tire marks show where vehicles have been thrown into hasty ninety-degree turns, followed by a second skid after a short stretch running west or east when the road heads north again onto the new meridian.