1. A method of generating a three-dimensional map of a region from successive images of the region captured from different camera poses, the map comprising a set of landmark points each defined by a three dimensional spatial position and image information associated with that three dimensional position, the method comprising: capturing successive images of the region; detecting, by a processor, a gravitational vertical direction in respect of each captured image; detecting, by the processor, feature points within the captured images; designating, by the processor, a subset of the captured images as a set of keyframes each having respective sets of image position data representing image positions of landmark points detected as feature points in that keyframe; in respect of a given image captured at a relocalisation operation: (i) deriving, by the processor, a camera pose for the given image from detected feature points in the given image; (ii) rotating, by the processor, the gravitational vertical direction in respect of the given image to the coordinates of a reference keyframe using the camera poses derived for the given image and the reference keyframe; and (iii) comparing, by the processor, the rotated gravitational vertical direction with an actual gravitational vertical direction associated with the reference keyframe so as to detect a quality measure of the given image.