1. A method of processing positioning data to generate map data which comprises a plurality of navigable segments representing segments of a navigable route in an area covered by the map data, speed data being associated with each segment, the method comprising: classifying the speed data associated with at least some navigable segments having speed data associated therewith into a category of a classification including a plurality of categories into which the speed data is classifiable according to a parameter of a navigable segment with which the speed data is associated; generating a category average for each category within the classification; assessing at least some navigable segments within the map data to determine whether there is speed data associated therewith; and using the category average, if there is no speed data associated with a navigable segment, according to the parameter of the navigable segment being processed, as the speed data for the navigable segment being processed.