ORB-SLAM is a feature-based simultaneous localization and mapping (SLAM) system. It has achieved good results in tracking, mapping and loop closing. However, the map created by ORB-SLAM with the monocular camera can not get the real scale. This paper presents an improving ORB SLAM system that helps to alleviate this issue by defining a baseline initialization procedure. We take two relative poses in the marker coordinate system to compute the real scale of the map in monocular mode. The hardware experimental platform is introduced and the algorithm is proposed in this paper. The localization method of ORB-SLAM is compared with the method of the laser scanner with the triangulation algorithm. And the maximum error is about 35cm in the range of 12 meters. The results of experiment show that the map created by ORB-SLAM with the monocular camera gets the real scale.