Principal Meridian
The north-south survey line through an initial point, run on the true meridian, from which ranges in the Public Land Survey System are numbered east and west.
Detailed Definition
A principal meridian is the north-south line of the Public Land Survey System (PLSS) that runs through an initial point. Ranges are numbered east and west from it, and every township described under it names it.
BLM's Manual of Surveying Instructions (2009) says a principal meridian "is intended to conform to the true meridian, extending north or south, or in both directions, from the initial point as conditions require" (section 3-8). True meridian means true north, not magnetic north. Federal law requires the public lands to be divided by "north and south lines run according to the true meridian" (43 U.S.C. 751).
What is set along it
Section and quarter-section corners are set alternately every 40 chains (half a mile) along a principal meridian, and township corners every 480 chains (six miles). Meander corners are set where the line meets a body of water large enough to be meandered.
How it is used
- Ranges are the six-mile columns of townships counted east and west of the principal meridian. R5E is the fifth column east.
- Townships are counted north and south of the base line, which crosses the principal meridian at the initial point.
- Guide meridians are run north from the base line and standard parallels every 24 miles east and west of the principal meridian (Manual section 3-14), because meridians converge toward the pole.
Each principal meridian has its own base line, so the words "and base line" are usually left off. BLM's specifications for land descriptions say the meridian's name "should be spelled in full."
Meridians governing the western mining states
BLM's Manual of Surveying Instructions (2009), table 3-1, lists the meridians that govern each state, wholly or in part:
- Alaska: Copper River, Fairbanks, Kateel River, Seward, and Umiat meridians
- Arizona: Gila and Salt River; Navajo
- California: Humboldt, Mount Diablo, and San Bernardino
- Colorado: Sixth Principal, New Mexico Principal, and Ute
- Idaho: Boise
- Montana: the Principal Meridian (its name)
- Nevada: Mount Diablo and San Bernardino. BLM Nevada notes that all but 7 townships in the state are on the Mount Diablo meridian.
- New Mexico: New Mexico Principal; Navajo
- Oregon and Washington: Willamette
- Utah: Salt Lake; Uintah Special
- Wyoming: Sixth Principal; Wind River
Why the meridian must be named
The same township and range numbers repeat under every meridian. "T. 3 N., R. 5 E." identifies nothing until the meridian is named, and a description with the wrong meridian points to ground that can be hundreds of miles away. California has three meridians and Nevada two, so a mining claim location, a lease, or a records search in either state has to state the meridian. Mining claim records in BLM's MLRS, master title plats, and county records are all indexed by meridian, township, range, and section.
Related Terms
PLSS
The Public Land Survey System, the rectangular survey system that divides the public lands into townships, ranges, and sections and serves as the basis for describing them.
Township, Range, and Section
The three parts of a Public Land Survey System land description: a 6-mile-square township located by township and range numbers, divided into 36 sections of nominally 640 acres.
Initial Point
The point where a principal meridian and its base line cross, serving as the origin for Public Land Survey System townships and ranges in a region.
Base Line
The east-west survey line through an initial point, run on a true parallel of latitude, from which townships in the Public Land Survey System are numbered north and south.