Principal Meridian, PLSS Definition

A principal meridian is a true north-south line — a meridian of longitude — that serves as the east-west reference datum for a PLSS survey area. Ranges are numbered by counting columns of townships east or west from the principal meridian. A description of "R4E" means the fourth column of townships east of the principal meridian; "R2W" means the second column to the west.

There are 37 named principal meridians in the contiguous United States and Alaska. Each covers a distinct geographic area. Some cover a single state (the Boise Meridian covers Idaho; the Salt Lake Meridian covers Utah and Nevada); others span multiple states (the Fifth Principal Meridian covers Arkansas, Iowa, Minnesota, Missouri, and North Dakota). The Sixth Principal Meridian, one of the most widely used, covers Colorado, Kansas, Nebraska, South Dakota, and Wyoming.

Principal meridians are not evenly spaced across the country. Their locations were chosen based on the pattern of westward settlement and the need to establish survey control in newly acquired territories. Early surveys in Ohio, Indiana, and Illinois used multiple meridians because the surveys were conducted piecemeal as settlement expanded. Later surveys in the West used larger, better-coordinated meridians that covered more territory.

The name of the principal meridian must be included in any complete PLSS legal description. A description that omits the meridian is ambiguous, because the same township and range numbers may exist in multiple states under different meridians. For example, "T5N, R3E" could refer to land under the Boise Meridian, the Willamette Meridian, or several others — the meridian name makes the description unique.