A latitudinal (east-west) line of survey, measured in degrees north or south of the equator (0° latitude), that runs perpendicular to a principal meridian and controls township designations in a PLSS survey. The base line intersects the principal meridian at the point of origin, or Initial Point, for the land survey. A principal meridian can have more than one controlling base line depending on the layout of the terrain covered by the meridian and the judgement of the surveyors (see extension of the Fourth Principal Meridian). A base line used to control township designations can also be referred to as a principal base line.
