In a modern mobile mapping system, the navigation component usually consists of a GPS receiver and an inertial navigation system (INS), which can provide accurate geo-referencing to the imaging sensors [1][2]. To achieve centimetre-level positioning accuracy, RTK-GPS is a natural choice. However, RTK-GPS in urban areas suffers from frequent outages due to blockages of either the GPS signals or the reference station radio links. In addition, multipath from buildings, trees and heavy vehicles could degrade the accuracy of the GPS when the vehicle is near them.