Section Line, PLSS Definition

Section lines are the interior boundary lines that divide a township into its 36 sections. They run in a grid pattern: north-south lines divide the township into six north-south columns, and east-west lines divide it into six rows. Together these create the 36 one-mile squares that are numbered as sections.

Section lines are one mile apart (in the standard interior portions of a township). The north-south section lines are not technically meridians — they are intended to be parallel to the principal meridian but are spaced one mile apart measured along the township's base line. The east-west section lines are parallel to the baseline. Because of Earth's curvature, the northern sections of a township are slightly narrower than the southern sections, with the accumulated difference assigned to the northernmost and westernmost sections.

In rural America, section lines have had a profound influence on road networks, property boundaries, and land use patterns. County roads in agricultural states like Iowa, Kansas, and Nebraska very commonly run along section lines, producing the characteristic grid of roads spaced one mile apart. Field boundaries, fence lines, and irrigation ditches are also frequently aligned with section lines because landowners built infrastructure along the survey grid when developing their land.

Section lines are physically marked in the field by section corners — monuments set at each intersection of section lines — and quarter-corner monuments set at the midpoints of each section boundary. These monuments, originally wooden posts or stone cairns but now typically aluminum caps set in concrete or iron rods, are the physical evidence of where the survey lines fall on the ground. When a boundary dispute arises, surveyors recover original section corners and quarter corners to establish the legal line.