*2.1. Coordinate Systems*

Two moving coordinate systems connected with the sensors were used in this paper: (1) The IMU coordinate system (ICS) with its origin at the center of the IMU, where the linear accelerations and angular rates are measured; and (2) The camera coordinate system (CCS) with its origin at the optical center of the camera and the z-axis aligned with the optical axis of the lens.

Another two reference coordinate systems were defined correspondingly: The static IMU coordinate system (SICS) with the same origin and axes as the ICS at the time of constructing the current keyframe; and the static camera coordinate system (SCCS) with the same origin and axes as the CCS at the time of constructing the current keyframe. The SICS and SCCS were static relative to the inertial space in the egomotion estimation.

In this paper, the superscript at the top-left corner of a vector represents the coordinate system the vector is expressed in. Specifically, <sup>I</sup> *p* represents the vector *p* in the ICS, <sup>C</sup>*p* represents the vector *p* in the CCS, SI*p* represents the vector *p* in the SICS, and SC*p* represents the vector *p* in the SCCS.
