A method of determining a longitudinal position of a vehicle relative to a digital map, the digital map comprising data representative of navigable elements of a navigable network along which the vehicle is travelling, the method comprising: determining real time scan data by scanning a lateral environment around the vehicle using at least one range-finder sensor, said real time scan data comprising one or more depth maps, each depth map representing the measured lateral distance to surfaces in the lateral environment for a plurality of longitudinal positions and elevations; retrieving localisation reference data associated with the digital map for a deemed current longitudinal position of the vehicle in relation to the digital map, wherein the localisation reference data comprises one or more depth maps, each depth map representing the lateral distance to surfaces in the lateral environment for a plurality of longitudinal positions and elevations, comparing the real time scan data to the localisation reference data by calculating a cross-correlation to determine a longitudinal offset between the real time scan data and the localisation reference data; and adjusting the deemed current longitudinal position based on said longitudinal offset to determine the longitudinal position of the vehicle relative to the digital map; characterized in that: the localisation reference data is provided as image data comprising one or more depth map images, and each pixel of each depth map image comprises: a first value indicative of a longitudinal position, a second value indicative of an elevation, and a third value indicative of a lateral position; determining real time scan data by scanning a lateral environment around the vehicle using at least one range-finder sensor, said real time scan data comprising one or more depth maps, each depth map representing the measured lateral distance to surfaces in the lateral environment for a plurality of longitudinal positions and elevations; retrieving localisation reference data associated with the digital map for a deemed current longitudinal position of the vehicle in relation to the digital map, wherein the localisation reference data comprises one or more depth maps, each depth map representing the lateral distance to surfaces in the lateral environment for a plurality of longitudinal positions and elevations, comparing the real time scan data to the localisation reference data by calculating a cross-correlation to determine a longitudinal offset between the real time scan data and the localisation reference data; and adjusting the deemed current longitudinal position based on said longitudinal offset to determine the longitudinal position of the vehicle relative to the digital map; the localisation reference data is provided as image data comprising one or more depth map images, and each pixel of each depth map image comprises: a first value indicative of a longitudinal position, a second value indicative of an elevation, and a third value indicative of a lateral position; a first value indicative of a longitudinal position, a second value indicative of an elevation, and a third value indicative of a lateral position;