Correction Line, PLSS Definition

The Earth is a sphere, and on a sphere, lines of longitude converge as they approach the poles. In the PLSS, range lines run true north-south, which means they are not parallel to each other — they gradually converge. Without correction, townships would become narrower and narrower moving away from the baseline, and the 6-mile-square ideal would be impossible to maintain.

Correction lines, also called standard parallels, address this problem by resetting the east-west reference at regular intervals. The General Land Office established standard parallels every 24 miles north and south of the baseline. At each standard parallel, new guide meridians are begun from surveyed points spaced exactly 24 miles apart, even though the previous guide meridians have converged slightly inward.

The result is that each 24-mile-square block of the survey grid is as geometrically consistent as possible. The accumulated convergence error is absorbed at each correction line rather than propagating indefinitely. In practice, this means that the north boundary of a township tier above a correction line is slightly wider than the south boundary of the tier below it — a small gap or overlap is introduced at the correction line itself.

When reading legal descriptions in areas far from the baseline, it is important to know which standard parallel governs the township in question. Surveyors working in states like Montana or Wyoming, where townships extend many hundreds of miles from the baseline, will encounter multiple correction lines. BLM plat maps show correction lines clearly, and understanding them helps explain why some townships contain slightly more or fewer than 640 acres in their northern or western tiers.