  Traditional fingerprint based localization techniques mainly rely on
infrastructure support such as RFID, Wi-Fi or GPS. They operate by war-driving
the entire space which is both time-consuming and labor-intensive. In this
paper, we present MLoc, a novel infrastructure-free localization system to
locate mobile users in a metro line. It does not rely on any Wi-Fi
infrastructure, and does not need to war-drive the metro line. Leveraging
crowdsourcing, we collect accelerometer,magnetometer and barometer readings on
smartphones, and analyze these sensor data to extract patterns. Through
advanced data manipulating techniques, we build the pattern map for the entire
metro line, which can then be used for localization. We conduct field studies
to demonstrate the accuracy, scalability, and robustness of M-Loc. The results
of our field studies in 3 metro lines with 55 stations show that M-Loc achieves
an accuracy of 93% when travelling 3 stations, 98% when travelling 5 stations.
