A rectangular survey has to start somewhere. In the PLSS it starts at an initial point, from which two control lines are run: a principal meridian running true north and south, and a base line running east and west along a parallel of latitude. Every township, range, section and quarter-quarter in that survey is counted outward from those two lines.
Thirty-seven principal meridians are conventionally enumerated for the United States, five of them in Alaska. Each governs its own region, and each is completely independent of the others. There is no continuous nationwide grid: the systems abut, and where they meet the townships do not line up.
The initial point and the two control lines
- Initial point
- The monumented point from which a principal meridian and its base line are run, and from which all townships and ranges in that survey system are numbered.
The initial point was fixed by astronomic observation, its latitude and longitude determined as accurately as the instruments of the day allowed, and monumented durably. From it the principal meridian was projected north and south on a true meridian, and the base line east and west on a true parallel of latitude. Both were monumented at every half mile: standard section corners at each mile and standard quarter-section corners at each half mile.
The distinction between those two lines is more than directional. A meridian is a great-circle line and can be run by holding a constant true bearing. A parallel of latitude is not a great circle. To run a base line the surveyor could not simply hold an east-west bearing, because a straight line started due east curves away from the parallel almost immediately. Base lines were therefore run by the solar method, by tangent offsets, or by the secant method, each of which is a technique for staying on a curved line while sighting along straight ones.
Numbering outward from the origin
Townships are counted north and south from the base line, and ranges east and west from the principal meridian. The first tier north of the base line is Township 1 North; the first range east of the meridian is Range 1 East. There is no township zero and no range zero, so the four quadrants around the initial point are distinguished by direction letters rather than by sign.
| Quadrant from the initial point | Township counted | Range counted |
|---|---|---|
| Northeast | North | East |
| Northwest | North | West |
| Southeast | South | East |
| Southwest | South | West |
A township therefore has a two-part address plus the name of the system: T3N, R2W, of the 6th Principal Meridian. The last part is not decoration. Without it the address is ambiguous, because T3N R2W exists under most of the thirty-seven meridians.
Why each system is independent
Principal meridians were not established on a plan. They were established as they were needed, as land was acquired and opened for settlement, over roughly a century. Some were laid out to serve a single land district; some serve most of a state or several states. The result is a patchwork whose boundaries follow the history of acquisition rather than any geometric logic.
Where two systems meet, the surveys were closed against each other. The townships of one system were run up to the boundary of the other and stopped there, producing closing corners on the governing line and fractional sections along it. Nothing about a township in one system implies anything about the township across the line in another. A retracement that crosses such a boundary is really two retracements, each governed by its own original survey and its own record.
Convergency: why the base line will not stay square
Meridians are not parallel. They converge toward the poles, so two range lines run true north from points six miles apart on the base line get closer together as they go. This is not a defect in the surveying; it is a property of the earth, and it is the reason the PLSS needs the correction mechanism of standard parallels and guide meridians.
θ ≈ d · tan φ / RFor two range lines six miles apart the convergency angle is only a few minutes of arc, but over a twenty-four mile run it accumulates to a visible amount: about 111 feet at latitude 30°, about 161 feet at latitude 40°, and about 221 feet at latitude 49°. That is why the framework of the PLSS is not one continuous grid but a series of blocks, each started fresh from a correction line.
What to take to the field
Principal meridians and base lines are surveyed lines with monuments, and those monuments are as much original evidence as any section corner. Because they were run first and run carefully, and because they control everything counted from them, their corners are frequently the best surviving control in a township. Standard corners along a base line or principal meridian are worth searching for even when your project lies several miles away, since a recovered standard corner can anchor a proportioning computation that would otherwise have no reliable end.
Equally, remember that the initial point's astronomic position was determined with the technology of its era. Modern geodetic coordinates for a monumented initial point often differ noticeably from the value originally published. The monument governs; the historical coordinate does not.