Localising portable apparatus

A method of localising portable apparatus in an environment comprises obtaining captured image data representing an image captured by an imaging device associated with the portable apparatus, and obtaining mesh data representing a 3-dimensional textured mesh of at least part of the environment. The mesh data is processed to generate a plurality of synthetic images, each 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. The plurality of synthetic images is analysed to find a said synthetic image similar to the captured image data, and an indication is provided of a pose of the portable apparatus within the environment based on the associated pose of the similar synthetic image. The mesh data may be processed to using distortion data representing the distortion produced by the lens of an imagining device and the similarity measurement may involve a minimum of a cost function such Normalised Information Distance (NID) possibly using a Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm.