There is disclosed a method of managing a database of positioning data, the positioning data including electromagnetic signal source data for use by a positioning system, and the method comprising: receiving signal data relating to signals received from a plurality of electromagnetic signal sources; associating an appropriate one of a plurality of zone identifiers with each of the electromagnetic signal sources, each zone identifier being associated with a respective geographical zone; selecting a subset of the plurality of electromagnetic signal sources in dependence on their associated zone identifiers; processing the signal data relating to the subset of the plurality of electromagnetic signal sources to compute position estimates of the electromagnetic signal sources; and updating the database of electromagnetic signal source data in dependence on the computed position estimates.