Section

A section is the unit of subdivision in the Public Land Survey System (PLSS): nominally one mile square and 640 acres. Every township is divided into 36 sections.

BLM's Glossary of Surveying and Mapping Terms defines a section as "the unit of subdivision of a township with boundaries conforming to the rectangular system of surveys, nominally one mile square, containing 640 acres." BLM's Manual of Surveying Instructions (2009) states: "Under the rectangular system, the unit of survey is the township of 36 sections. The unit of subdivision is the section of 640 acres" (section 3-2).

The legal basis

Federal law requires each township to be subdivided into sections "containing, as nearly as may be, six hundred and forty acres each, by running parallel lines through the same from east to west and from south to north at the distance of one mile from each other, and marking corners at the distance of each half mile" (43 U.S.C. 751).

How sections are numbered

Numbering begins with 1 in the northeast section, "proceeding thence west to section 6, thence south to section 7, thence east to section 12, and so on, alternately, to number 36 in the southeast section" (Manual section 3-3). In a full township:

• Top row, east to west: 1 to 6
• Second row, west to east: 7 to 12
• The rows alternate down to the bottom row, 31 to 36, with 31 in the southwest and 36 in the southeast

Regular and irregular sections

The Manual says a regular township contains 25 regular sections returned as 640 acres each. The sections along the north and west boundaries take up the township's excess or deficiency in measurement. Each of them except section 6 has aliquot parts totaling 480 acres plus four lots, and section 6 has 360 acres of aliquot parts plus seven lots (section 3-32).

The glossary defines a regular section as one whose boundaries, as returned on the original survey, are within 21 minutes of cardinal direction and within 25 links of each 40 chains of measurement. Sections cut by a meanderable body of water or by other segregated land are divided into as many aliquot parts as possible and then lots (section 3-106).

Contents are as returned

By law, "the contents of each section or subdivision of section returned shall be held and considered as containing the exact quantity expressed" (Manual section 3-4, citing 43 U.S.C. 752). The acreage on the official plat controls.

Why it matters for mining claims

The section is the everyday unit of mining claim research. BLM's MLRS, master title plats, and county records are indexed by meridian, township, range, and section, and a lode mining claim can cross section lines. In a regular township, sections 1 through 7, 18, 19, 30, and 31 border the north or west boundary, so expect lots there rather than full quarter sections.