Abstract: 1-D calibration is an important camera calibration method.In the literatures, almost all the 1-D calibration methods need the 1-D calibration object to move within a space plane.As a result, the 1-D calibration object, before and after the planar motion, is equivalent to a 2-D calibration object. From the point of projective geometry, as the homography between the image plane and object plane encapsulates all the information of the intrinsic and extrinsic parameters of the camera, those constraints of 1-D calibration, from various methods in the literatures, could be also derived from homography.In this work, 1-D calibration is reinvestigated via the homography, and equivalent constraints are obtained.