Baseline, PLSS Definition

A baseline is the east-west line that serves as the latitudinal foundation for a PLSS survey area. Every principal meridian in the United States has exactly one associated baseline. Together, these two lines form the initial point from which the entire rectangular survey grid for that region is measured.

Townships are numbered by counting rows north or south of the baseline. A township described as "T4N" lies in the fourth row of townships north of the baseline. "T2S" lies in the second row south. This counting system applies uniformly across the entire survey area tied to that baseline, giving surveyors and landowners an unambiguous reference.

Baselines are true parallels of latitude, meaning they follow the curvature of the Earth. Because of this curvature, north-south range lines that start perpendicular to the baseline will eventually converge as they extend north. To manage this convergence and keep the grid roughly square, the PLSS uses standard parallels (correction lines) at regular intervals north and south of the baseline.

Not every baseline aligns with a state boundary or other recognizable geographic feature. The Humboldt Baseline in California, for example, runs through the northern part of the state, while the Mount Diablo Baseline covers most of the central and southern portions. Knowing which baseline governs a particular survey area is the first step in locating land on historical maps and county records.