1. A method of generating a three-dimensional map of a region from successive images of the region captured from different camera positions, 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, by an imaging device, successive images of the region; detecting, by a 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 measurement data representing image positions of landmark points detected as feature points in that image, each keyframe being connected to at least one other keyframe in the set by a connection comprising at least a threshold number of landmark points, having corresponding measurement data for that keyframe, in common with the same landmark points having corresponding measurement data for that other keyframe in the set, and the set of keyframes being such that each keyframe in the set is directly or indirectly connected to all of the other keyframes in the set by a sequence of one or more such connections; detecting, by the processor, whether candidate measurement data associated with a given one of the keyframes may be removed from the set of measurement data for that keyframe, by detecting whether each keyframe, in the set of keyframes without the candidate measurement data, is still directly or indirectly connected to all of the other keyframes in the set of keyframes by a sequence of one or more connections; and if so, removing that candidate measurement data.