The estimated horizontal accuracy is a measure of the confidence
on the location reported by the GPS sensor; it is a component of a
GPS sensor report (see Definition 1). The accuracy is reported in
meters. Different transportation modes should have different
estimated accuracies. For example, traveling by aboveground trains
should have worse accuracy than walking, since walking has a
clearer view of the GPS satellites in the sky. Additionally, we
consider the average accuracy for a set of GPS reports as a feature, instead of the instantaneous accuracy as done in [14]. Taking the
average accuracy is more realistic since the GPS system may
introduce uncertainties.