1. A method of processing positioning data to generate map data within a map, the map comprising a plurality of navigable segments representing segments of a navigable route in an area covered by the map associated with each segment, the method comprising: generating a measured speed profile for each navigable segment from received trace data, the measured speed profile being representative of a set of average speeds for each of a plurality of time periods for the navigable segment; classifying each navigable segment into one of a plurality of predetermined categories according to a parameter of the navigable segment; generating a category average speed for each category within the classification; for each of one or more of the navigable segments, performing a quality assessment of the measured speed profile for the navigable segment to determine if the measured speed profile is of a predetermined quality; when the measured speed profile is of the predetermined quality, using the measured speed profile as the speed data for the navigable segment; when the measured speed profile is not of the predetermined quality, determining if an average of speed data for the navigable segment meets a first threshold; when the average of the speed data meets the first threshold, generating a replacement speed profile for the navigable segment based on the average of the speed data and using the replacement speed profile as the speed data for the navigable segment; when the average of the speed data does not meet the first threshold, using the corresponding category average speed as the speed data for the navigable segment, wherein using the category average speed comprises determining that the category average speed meets a second threshold; and processing the speed data associated with each navigable segment to generate said map data.