A method of determining a 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: storing localisation reference data associated with the digital map, wherein the localisation reference data comprises at least one depth map image in the form of a raster image, wherein the at least one depth map image is indicative of an environment around the vehicle projected on to a reference plane, the reference plane being defined by a reference line associated with the navigable element, each pixel of the at least one depth map image: being associated with a longitudinal position and an elevation in the reference plane associated with the navigable element along which the vehicle is travelling, and including a depth channel representing a distance to a surface of an object in the environment along a predetermined direction from the associated position of the pixel in the reference plane; retrieving, from the stored localisation data, localisation reference data associated with the digital map for a deemed current position of the vehicle along the navigable element of the navigable network; using the at least one depth map image of the retrieved localisation reference data for the deemed current position of the vehicle along the navigable element of the navigable network to determine a reference point cloud indicative of the environment around the vehicle, the reference point cloud including a set of first data points in a three-dimensional coordinate system, wherein each first data point represents a surface of an object in the environment; determining real time scan data by scanning the environment around the vehicle using at least one sensor of the vehicle, the real time scan data comprising a point cloud indicative of the environment around the vehicle, the point cloud including a set of second data points in the three-dimensional coordinate system, wherein each data point represents a surface of an object in the environment as determined using the at least one sensor; calculating a correlation between the point cloud of the real time scan data and the point cloud determined from the at least one depth map image to determine an alignment offset between the point clouds; and using the determined alignment offset to adjust the deemed current position to determine the position of the vehicle relative to the digital map. storing localisation reference data associated with the digital map, wherein the localisation reference data comprises at least one depth map image in the form of a raster image, wherein the at least one depth map image is indicative of an environment around the vehicle projected on to a reference plane, the reference plane being defined by a reference line associated with the navigable element, each pixel of the at least one depth map image: being associated with a longitudinal position and an elevation in the reference plane associated with the navigable element along which the vehicle is travelling, and including a depth channel representing a distance to a surface of an object in the environment along a predetermined direction from the associated position of the pixel in the reference plane; being associated with a longitudinal position and an elevation in the reference plane associated with the navigable element along which the vehicle is travelling, and including a depth channel representing a distance to a surface of an object in the environment along a predetermined direction from the associated position of the pixel in the reference plane; retrieving, from the stored localisation data, localisation reference data associated with the digital map for a deemed current position of the vehicle along the navigable element of the navigable network; using the at least one depth map image of the retrieved localisation reference data for the deemed current position of the vehicle along the navigable element of the navigable network to determine a reference point cloud indicative of the environment around the vehicle, the reference point cloud including a set of first data points in a three-dimensional coordinate system, wherein each first data point represents a surface of an object in the environment; determining real time scan data by scanning the environment around the vehicle using at least one sensor of the vehicle, the real time scan data comprising a point cloud indicative of the environment around the vehicle, the point cloud including a set of second data points in the three-dimensional coordinate system, wherein each data point represents a surface of an object in the environment as determined using the at least one sensor; calculating a correlation between the point cloud of the real time scan data and the point cloud determined from the at least one depth map image to determine an alignment offset between the point clouds; and using the determined alignment offset to adjust the deemed current position to determine the position of the vehicle relative to the digital map.