Standard Parallel, PLSS Definition

Standard parallels are true parallels of latitude — east-west lines that run at exactly the same latitude as they extend across the survey area. In the PLSS, standard parallels are established at 24-mile intervals north and south of the baseline. They are also called correction lines because their primary function is to correct the accumulated error caused by converging meridians.

Because range lines are true meridians and meridians converge toward the poles, the north end of any 24-mile-tall block of townships is narrower than the south end. Without correction, this convergence would cause the grid to become increasingly distorted moving away from the baseline. Standard parallels interrupt this distortion: at each standard parallel, new guide meridians begin from points measured exactly 24 miles apart along the parallel, resetting the east-west spacing regardless of how much convergence has occurred below.

The result is a slight "jog" at each standard parallel. The guide meridian that continues north from a standard parallel is typically offset slightly to the east relative to where the range line below ended. This offset — usually a matter of a few feet to a few hundred feet depending on the latitude — is absorbed at the standard parallel and does not propagate further. On maps and aerial photographs, these jogs appear as breaks in otherwise continuous north-south roads and property lines.

Standard parallels are numbered by their distance from the baseline: the First Standard Parallel North is 24 miles north of the baseline, the Second Standard Parallel North is 48 miles north, and so on. In states that extend far from their baselines — Montana, for example, spans multiple standard parallels — the parallel number is important context for interpreting survey plats and field notes. BLM plat maps clearly show standard parallels and the guide meridians that begin at them.