Abstract: Provided are a mobile device (100) and computer-implemented method (700) for localisation in an existing map of a 3-D environment. For a first image frame, a first pose is localised based on visual features. For a second image frame, a pose is predicted (810) based on inertial measurements, combined with the pose of the first image frame. Based on the predicted pose, the method predicts (830) a set of landmarks that are likely to be visible. A second pose is then calculated (850), for the second image frame, based on matching (840) visual features of the second image frame to the set of landmarks.