GNSS positioning is hard to be used in indoor environments. Thus, many indoor positioning approaches have been developed, such as Wi-Fi positioning, Beacon positioning, IMES positioning, visible light positioning, and geomagnetic positioning. Moreover, indoor-outdoor seamless positioning approach is also discussed for location-based services in urban areas. However, there are various technical issues, such as positioning accuracy, device cost, and smooth positioning mode switching between outdoor and indoor environments. In this study, we proposed an indoor-outdoor seamless positioning methodology with single-frequency GNSS single positioning and fingerprint-based geomagnetic positioning. In our experiments, we implemented a low-cost and low-power GNSS receiver and geomagnetic sensor on a single board computer. Through our experiments, our methodology can achieve positioning among indoor and outdoor environments.