A computer-implemented method of localising portable apparatus in an environment, the method comprising: obtaining captured image data representing an image captured by an imaging device fitted to/on the portable apparatus at run time; obtaining mesh data representing a 3-dimensional textured mesh of at least part of the environment; processing the mesh data to generate a plurality of synthetic images, each said synthetic image being associated with a pose within the environment and being a simulation of an image that would be captured by the imaging device from that associated pose; analysing the plurality of synthetic images to find a said synthetic image similar to the captured image data, and providing an indication of a pose of the portable apparatus within the environment based on the associated pose of the similar synthetic image, the method characterized in that the step of processing the mesh data comprises: obtaining distortion data representing a distortion produced by a lens of the imaging device, and using the distortion data to generate each said synthetic image, and wherein the lens of the imaging device comprises a wide angle lens, and wherein the step of using the distortion data comprises: mapping undistorted image coordinates of the synthetic image to distorted image coordinates of the synthetic image based on the distortion data using a B-spline interpolation process to interpolate a look-up table that represents a mapping of the undistorted image coordinates to the distorted image coordinates. obtaining captured image data representing an image captured by an imaging device fitted to/on the portable apparatus at run time; obtaining mesh data representing a 3-dimensional textured mesh of at least part of the environment; processing the mesh data to generate a plurality of synthetic images, each said synthetic image being associated with a pose within the environment and being a simulation of an image that would be captured by the imaging device from that associated pose; analysing the plurality of synthetic images to find a said synthetic image similar to the captured image data, and providing an indication of a pose of the portable apparatus within the environment based on the associated pose of the similar synthetic image, the method characterized in that the step of processing the mesh data comprises: obtaining distortion data representing a distortion produced by a lens of the imaging device, and using the distortion data to generate each said synthetic image, and wherein the lens of the imaging device comprises a wide angle lens, and wherein the step of using the distortion data comprises: mapping undistorted image coordinates of the synthetic image to distorted image coordinates of the synthetic image based on the distortion data using a B-spline interpolation process to interpolate a look-up table that represents a mapping of the undistorted image coordinates to the distorted image coordinates. obtaining distortion data representing a distortion produced by a lens of the imaging device, and using the distortion data to generate each said synthetic image, and wherein the lens of the imaging device comprises a wide angle lens, and wherein the step of using the distortion data comprises: mapping undistorted image coordinates of the synthetic image to distorted image coordinates of the synthetic image based on the distortion data using a B-spline interpolation process to interpolate a look-up table that represents a mapping of the undistorted image coordinates to the distorted image coordinates.