A method of determining extrinsic calibration parameters for at least one pair of sensing devices mounted on transportable apparatus, the method including: obtaining image data representing a plurality of images captured by an image generating sensing device of the pair at a series of poses during motion through an environment of transportable apparatus; obtaining data representing a 3D point cloud based on data produced by a 2D LIDAR sensing device of the pair during the motion through the environment of the transportable apparatus, wherein the 3D point cloud is created using a trajectory estimate obtained using an Inertial Navigation System of the transportable apparatus or visual odometry; selecting a said image from the plurality of images of the image data, the image being captured by the image generating sensing device at a particular said pose of the series of poses; the method characterised by: estimating a solution for full SE3 transformation between the 2D LIDAR sensing device and the image generating sensing device to generate (208, 210) a laser reflectance image based on a portion of the 3D point cloud data corresponding to the particular pose of the image generating sensing device; performing patch-based normalisation of the selected image and the laser reflectance image; using an iterative grid-based optimisation technique to search over the poses that maximise alignment of the selected image and the laser reflectance image; computing a metric using an edge-based, weighted Sum of Squared Difference, SSD, objective function to measure the alignment between the selected image and the corresponding laser reflectance image, and using the metric to determine extrinsic calibration parameters for at least one of the sensing devices. obtaining image data representing a plurality of images captured by an image generating sensing device of the pair at a series of poses during motion through an environment of transportable apparatus; obtaining data representing a 3D point cloud based on data produced by a 2D LIDAR sensing device of the pair during the motion through the environment of the transportable apparatus, wherein the 3D point cloud is created using a trajectory estimate obtained using an Inertial Navigation System of the transportable apparatus or visual odometry; selecting a said image from the plurality of images of the image data, the image being captured by the image generating sensing device at a particular said pose of the series of poses; the method characterised by: estimating a solution for full SE3 transformation between the 2D LIDAR sensing device and the image generating sensing device to generate (208, 210) a laser reflectance image based on a portion of the 3D point cloud data corresponding to the particular pose of the image generating sensing device; performing patch-based normalisation of the selected image and the laser reflectance image; using an iterative grid-based optimisation technique to search over the poses that maximise alignment of the selected image and the laser reflectance image; computing a metric using an edge-based, weighted Sum of Squared Difference, SSD, objective function to measure the alignment between the selected image and the corresponding laser reflectance image, and using the metric to determine extrinsic calibration parameters for at least one of the sensing devices. estimating a solution for full SE3 transformation between the 2D LIDAR sensing device and the image generating sensing device to generate (208, 210) a laser reflectance image based on a portion of the 3D point cloud data corresponding to the particular pose of the image generating sensing device; performing patch-based normalisation of the selected image and the laser reflectance image; using an iterative grid-based optimisation technique to search over the poses that maximise alignment of the selected image and the laser reflectance image; computing a metric using an edge-based, weighted Sum of Squared Difference, SSD, objective function to measure the alignment between the selected image and the corresponding laser reflectance image, and using the metric to determine extrinsic calibration parameters for at least one of the sensing devices.