  In this paper, we introduce a method for visual relocalization using the
geometric information from a 3D surfel map. A visual database is first built by
global indices from the 3D surfel map rendering, which provides associations
between image points and 3D surfels. Surfel reprojection constraints are
utilized to optimize the keyframe poses and map points in the visual database.
A hierarchical camera relocalization algorithm then utilizes the visual
database to estimate 6-DoF camera poses. Learned descriptors are further used
to improve the performance in challenging cases. We present evaluation under
real-world conditions and simulation to show the effectiveness and efficiency
of our method, and make the final camera poses consistently well aligned with
the 3D environment.
