A computerised method of creating map data from a plurality of trips where each trip comprises position data derived from the positions of at least one navigation device 200 over a period of time, the method comprising using a processing circuitry to perform the following steps: i. processing the position data; ii. determining from the processing the believed accuracy of the position data constituting a trip; iii. reducing the weight applied to and/or removing position data in which the belief is below a predetermined threshold to generate filtered position data; iv. deriving information about the road 700 based upon the filtered position data; and v. generating further map data.