1. The reference line in a measurement by triangulation. In meteorological observations it has several applications, for example, 1) the horizontal distance from the observation point to the location of a ceiling-light projector; 2) the horizontal distance between a ceilometer projector and detector; and 3) the bearing, distance, and slope of the line between the observational points in a double-theodolite observation. 2. In dual-Doppler analysis, the length and bearing of the line separating the radars.