  The scale ambiguity problem is inherently unsolvable to monocular SLAM
without the metric baseline between moving cameras. In this paper, we present a
novel scale estimation approach based on an object-level SLAM system. To obtain
the absolute scale of the reconstructed map, we derive a nonlinear optimization
method to make the scaled dimensions of objects conforming to the distribution
of their sizes in the physical world, without relying on any prior information
of gravity direction. We adopt the dual quadric to represent objects for its
ability to fit objects compactly and accurately. In the proposed monocular
object-level SLAM system, dual quadrics are fastly initialized based on
constraints of 2-D detections and fitted oriented bounding box and are further
optimized to provide reliable dimensions for scale estimation.
