Map building is a foundational task in the navigation area of the mobile robot.A new algorithm named middle line of free space（MLFS） is proposed.Laser sensor is used to scan data and to built up metric map

environmental boundary on the basis of constructing metric map

divide scanned datas into four parts using robot as center.With the proposed algorithm

the topological map in the free space of the environment is extracted. Experiments show that this algorithm can effectively

real-time and compactly represent environment.