Method for calculating the distance travelled by a vehicle

The method involves extracting set of readings of consecutive positions (p-1-p-3,p-i), and disregarding those position readings. A set quality index is determined based on position readings of set or based on position quality indices. A path length is determined based on mutual distances of consecutive position readings of set. The path length is determined using a digital road map (1') by associating position readings of set with road segments (S-1-S-4) having respective known segment lengths. The path length is determined based on segment length of the road segment. An independent claim is included for an onboard unit carried in vehicle.