A next-generation digital road map (DRM) is expected to be an effective technology for achieving detailed navigation and driving safety assistance. As certain precise road information is required to make the next-generation DRM, the mobile mapping system (MMS) has attracted considerable attention as an efficient and precise survey method to gather such information. Since the MMS is a vehicle equipped with a GPS/inertial measurement unit along with a navigation system, cameras, and laser scanners, it can acquire precise, detailed 3D road information. However, when the MMS surveys are carried out during a GPS blockage, the positioning precision is reduced. In such a case, the MMS corrects its position by observing landmarks (LMs) which is surveyed its position by conventional method to calculate the precise position. In this study, we performed simulations of the positioning correction method to evaluate optimum landmark intervals and estimated the acceptable observation errors. We also conducted a field test to evaluate the effectiveness of the abovementioned positioning correction method.