3D Decision Aid System for City Planning is a set of true 3D virtual reality system researched and developed using digital survey and mapping and computer technology and in combination with features of survey and mapping database. Compared with ordinary display system, the system functions both e-map and e-sand-plate and can perform target query, observation and analysis. In addition, it has also the function of virtual construction so as to perform its 3D virtual design on basis of digital map.