A method generates a three-dimensional map of a region from successive images of that region captured from different camera poses. The method captures successive images of the region, detects a gravitational vertical direction in respect of each captured image, detects feature points within the captured images and designates 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 image. The method also includes, for a captured image (i) deriving a camera pose from detected feature points in the image; (ii) rotating the gravitational vertical direction to the coordinates of a reference keyframe using the camera poses derived for that image and the reference keyframe; and (iii) comparing the rotated direction with the actual gravitational vertical direction for the reference keyframe to detect a quality measure of that image.