A method and system for tracking mobile objects in a site are disclosed. The system comprises a computer cloud communicating with one or more imaging devices and one or more tag devices. Each tag device is attached to a mobile object, and has one or more sensors for sensing the motion of the mobile object. The computer cloud visually tracks mobile objects in the site using image streams captured by the imaging devices, and uses measurements obtained from tag devices to resolve ambiguity occurred in mobile object tracking. The computer cloud uses an optimization method to reduce power consumption of tag devices.