Wikipedia
Section lines in the United States are one mile (1.6 km) apart. When surveyors originally mapped an area, for instance a township, it was their custom to divide the new township into 36 . Property ownership often followed this layout. A section is a area. A half section is a 1/2-mile by area. It is proper to continue this division down to a 1/4-by-1/4 section which is 1/16 of a section, or . The next smaller division is , and then . Besides property ownership, roads called section line roads often followed the section lines, and one can often still see them in modern maps, even in urban areas. In rural areas, these roads are called section roads, and often exist primarily so that farmers can access their land.
Usage examples of "section line".
He stopped half a mile short of Cacher, turned directly north on to a section line road, and drove north at a hundred miles an hour, turning up a rooster-tail of yellowish lead-saturated dust.
He went quietly to talk to Yolanda, where she sat with her mother and sister in the new sitting room, the first one across the section line.
And if the cross-section line intersects some Flatland object, then A Square is seeing the innards of that object.