A Novel Ship-Height Measurement System Based on Optical Axes Collinear Stereo Vision

Overheight ships pose serious danger to bridges when sailing along inland rivers. Measuring ship height from a long distance with high precision is a very challenging problem. This article proposes a novel optical axes colinear binocular stereo vision system to meet this goal. The system is equipped with two arrays of cameras, each consisting of a long-focus camera (LFC) and a short-focus camera (SFC), located face-to-face on each side of a river. The SFC, i.e., wide-angle-lens camera, is used for ship detection, while the LFC, i.e., tele lens camera, is used for precise ship-height measurement. Different from conventional binoculars, the optical axis of the two LFCs are arranged to be colinear in our setup. The advantage of this configuration is in twofold. First, it is simple and precise to calibrate the measuring apparatus and the system error since the two LFCs are mutually visible to each other. Second, due to its face-to-face configuration across the river, the entire navigation waterway is under constant monitoring without blind spot; hence, the shortcoming of limited field of view (FOV) of tele lens is overcome with conventional parallel optical axis stereo vision setup. In this work, the detailed error analysis was performed and in-field experiments were well designed. The experimental data matched very well with the theoretical prediction. The proposed system can achieve a ship-height measuring error below 0.1 m for a distance of 1.46 km.