1. A method of deriving a position coordinate estimate of a receiver from a plurality of parameter measurements, each of the plurality of parameter measurements concurrently measured at the receiver, each of the plurality of parameter measurements derived from a correlation function which in turn is derived by correlating a signal received at the receiver with an identification code uniquely identifying a transmitter visible to the receiver, comprising: deriving a plurality of different position coordinate estimates of a current position of the receiver based on the plurality of parameter measurements, each of the plurality of parameter measurements related to a distance between the receiver and an associated transmitter, at least two of the plurality of different position coordinate estimates of the current position of the receiver being derived with the same type of position-determination scheme from a different subset of four or more of the plurality of parameter measurements; and selecting, based on local topography, one of the plurality of different position coordinate estimates.