Hitherto it has been the practice to compute the altitude and azimuth of a celestial body either for a D.R. position or a chosen position in the vicinity of the D.R. position and to compare the computed altitude with the observed altitude, the difference (the intercept) being marked off along a line drawn in the direction of the computed azimuth from the position used for computing, either ‘from’ or ‘towards’ the object; a perpendicular drawn at the end of the intercept is the position line of Marcq. St. Hilaire.