(1) A survey line used in the government survey to establish township lines. The base line runs east and west through a principal meridian (line running north and south). (2) A horizontal elevation line used as the centerline in a survey for a highway route.
(1) A surveyed line of great accuracy which serves as the basis for measurement of other lines or of the angles between them. (2) In a land survey system an East-West line passing through the origin from which township, section, and quarter-section corners are established.
The base line of a projection refers to the period of population change that is being extrapolated.