Abstract: Methods and systems employ at least two motion estimators to form respective estimates of position of a mobile device over time. The estimates of position over time are based on sensor data generated at the mobile device. Each motion estimator is associated with a respective reference frame, and each respective estimate of position includes one or more estimate components. A transformation from the reference frame associated with a second motion estimator to the reference frame associated with a first motion estimator is determined. The transformation is determined based at least in part on at least one estimate component of the one or more estimate components of the estimates of position formed by each of the first and second motion estimators.