A method of generating localisation reference data associated with a digital map, the localisation reference data providing a compressed representation of an environment around at least one navigable element of a navigable network represented by the digital map, the method comprising: obtaining, for at least one navigable element represented by the digital map, a set of data points in a three-dimensional coordinate system, wherein each data point represents a surface of an object in the environment around the at least one navigable element of the navigable network; generating, from the set of data points, localisation reference data comprising at least one depth map indicative of the environment around the navigable element projected on to a reference plane, said reference plane being defined by a reference line associated with the navigable element, each pixel of the at least one depth map being associated with a position in the reference plane associated with the navigable element, and the pixel including a depth channel representing the 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, wherein the distance to the surface of an object represented by the depth channel of each pixel is determined based upon a group of a plurality of sensed data points, each indicative of a sensed distance to the surface of an object along the predetermined direction from the position of the pixel, and wherein the distance to the surface of the object represented by the depth channel of the pixel is the closest distance, or closest mode distance obtained using a distribution of sensed distances, based on the group of sensed data points; and associating the generated localisation reference data with the digital map data. obtaining, for at least one navigable element represented by the digital map, a set of data points in a three-dimensional coordinate system, wherein each data point represents a surface of an object in the environment around the at least one navigable element of the navigable network; generating, from the set of data points, localisation reference data comprising at least one depth map indicative of the environment around the navigable element projected on to a reference plane, said reference plane being defined by a reference line associated with the navigable element, each pixel of the at least one depth map being associated with a position in the reference plane associated with the navigable element, and the pixel including a depth channel representing the 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, wherein the distance to the surface of an object represented by the depth channel of each pixel is determined based upon a group of a plurality of sensed data points, each indicative of a sensed distance to the surface of an object along the predetermined direction from the position of the pixel, and wherein the distance to the surface of the object represented by the depth channel of the pixel is the closest distance, or closest mode distance obtained using a distribution of sensed distances, based on the group of sensed data points; and associating the generated localisation reference data with the digital map data. <EOS>