A method of localising transportable apparatus within an environment, the method including: receiving data obtained from a 2-dimensional, 2D, laser scanner of the transportable apparatus configured to collect information relating to at least a surface over which the transportable apparatus is moving in an environment, the 2D laser scanner further comprising a clock configured to provide timing information relating to the information relating to at least the surface; using the 2D laser scanner data and reconstructing motion of the transportable apparatus over a period when the 2D laser scanner data was created, using information regarding linear and rotational velocities of the transportable apparatus provided during the period, to generate a new 3D point cloud of the environment; obtaining data representing an existing 3D point cloud of at least part of the environment, seeking to match the new 3D point cloud with, or within, the existing 3D point cloud in order to localise the transportable apparatus with respect to the existing point cloud, wherein the information regarding rotational velocity is obtained from a rotational velocity measuring device, the rotational velocity measuring device including a clock to provide timing information relating to the rotational velocity information, and the information regarding linear velocity is obtained from a linear velocity measuring device configured to provide information relating to linear velocity of the transportable apparatus, the linear velocity measuring device including a clock to provide timing information relating to the linear velocity information, wherein the method includes calibrating the clock of the 2D laser scanner, the clock of the linear velocity measuring device and the clock of the rotational velocity measuring device in order to coordinate information provided by those devices in terms of timing, characterised in that the method further includes adjusting the velocity information provided by the linear velocity measuring device in view of a bias factor wherein computing the bias factor includes: scaling observed velocity information over a test section of the environment; generating a scaled 3D point cloud for the test section; attempting to align the scaled 3D point cloud with a part of the existing 3D point cloud corresponding to the test section, and using information relating to the alignment to compute the bias factor. receiving data obtained from a 2-dimensional, 2D, laser scanner of the transportable apparatus configured to collect information relating to at least a surface over which the transportable apparatus is moving in an environment, the 2D laser scanner further comprising a clock configured to provide timing information relating to the information relating to at least the surface; using the 2D laser scanner data and reconstructing motion of the transportable apparatus over a period when the 2D laser scanner data was created, using information regarding linear and rotational velocities of the transportable apparatus provided during the period, to generate a new 3D point cloud of the environment; obtaining data representing an existing 3D point cloud of at least part of the environment, seeking to match the new 3D point cloud with, or within, the existing 3D point cloud in order to localise the transportable apparatus with respect to the existing point cloud, wherein the information regarding rotational velocity is obtained from a rotational velocity measuring device, the rotational velocity measuring device including a clock to provide timing information relating to the rotational velocity information, and the information regarding linear velocity is obtained from a linear velocity measuring device configured to provide information relating to linear velocity of the transportable apparatus, the linear velocity measuring device including a clock to provide timing information relating to the linear velocity information, wherein the method includes calibrating the clock of the 2D laser scanner, the clock of the linear velocity measuring device and the clock of the rotational velocity measuring device in order to coordinate information provided by those devices in terms of timing, characterised in that the method further includes adjusting the velocity information provided by the linear velocity measuring device in view of a bias factor wherein computing the bias factor includes: scaling observed velocity information over a test section of the environment; generating a scaled 3D point cloud for the test section; attempting to align the scaled 3D point cloud with a part of the existing 3D point cloud corresponding to the test section, and using information relating to the alignment to compute the bias factor. scaling observed velocity information over a test section of the environment; generating a scaled 3D point cloud for the test section; attempting to align the scaled 3D point cloud with a part of the existing 3D point cloud corresponding to the test section, and using information relating to the alignment to compute the bias factor.