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.
Detailed Definition
A base line is the east-west line of the Public Land Survey System (PLSS) that runs through an initial point and crosses the principal meridian there. Townships are numbered north and south from it.
BLM's Manual of Surveying Instructions (2009) says: "The base line is extended east and west from the initial point on a true parallel of latitude" (section 3-10). A parallel of latitude is not a straight line on the ground. It curves slightly, staying the same distance from the equator along its whole length.
What is set along it
Standard section and quarter-section corners are set alternately every 40 chains (half a mile), and standard township corners every 480 chains (six miles). They are "standard" corners because they sit on the base line or a standard parallel and control the surveys north of it.
How it is used
- Townships are the six-mile rows counted north and south of the base line. T3N is the third row north.
- Ranges are counted east and west of the principal meridian.
- A township and range together, with the meridian named, identify one six-mile-square township.
Each principal meridian has its own base line, which is why land descriptions name only the meridian.
Standard parallels (correction lines)
Range lines run north toward the pole, so they draw closer together as they go. The Manual explains that the range lines "would converge and intersect at the North Pole," and that "the convergency is taken up at intervals by the running of standard parallels, on which the measurements are again made full" (section 1-30).
Standard parallels are run east and west "at intervals of 24 miles north and south of the base line, in the manner prescribed for the survey of the base line" (section 3-13). They were first called correction lines. The Manual notes that older surveys often placed them 30 or 36 miles apart.
Offsets in the grid
Because townships are measured full again on each standard parallel, range lines jog where they meet one. The Manual describes "offsets in the range lines and two sets of corners, standard corners for the lines to the north and closing corners for lines to the south." These jogs show clearly on township plats and on maps of mining claims.
Why it matters in research
Townships near a base line or standard parallel often have irregular sections and lots, and two sets of corners along one line. A mining claim or lease described by legal subdivision in such a township has to be read against the official plat, not an idealized grid.
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.
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.