Public Land Survey System and Original Government Surveys
The grid
A principal meridian and base line anchor the system. Townships are nominally 6 miles square, numbered north from the base line and ranges east or west from the meridian, each containing 36 sections numbered from the northeast corner in a boustrophedon pattern. Standard parallels (correction lines) run every 24 miles, with guide meridians closing the blocks; closing corners and double corners appear where later surveys meet earlier ones.
Subdividing a section
Regular subdivision proceeds by running straight lines between opposite quarter-section corners, then between quarter-quarter corners. Aliquot parts such as the SW 1/4 of the NE 1/4 are described hierarchically. Fractional sections along meander lines, navigable water, and correction lines produce numbered lots rather than aliquots. The GLO plat and field notes are the record of the original survey and the source of bearing and distance evidence.
Traps in questions
Assuming every section is exactly 640 acres or one mile square; treating a meander line as a boundary; using the plat as a boundary survey of record; forgetting that aliquot parts are measured proportionally when corners are lost; confusing the 1/4 corner with a midpoint of the section line without checking the record.
Worked example
For the NE 1/4 of the SE 1/4 of a section, build from the section corners: locate the 1/4 corners, connect them to find the center of the section, then halve the SE 1/4 and halve again. The nominal area is 40 acres, but compute the actual area from the record distances between the controlling corners, since the original section may not close at 6,400 feet.