1. A method of detecting potential changed objects in a first image and a second image, where the first and second images at least partially overlap, the method including: obtaining data describing a first image and data describing a second image; detecting a first set of objects in the first image data; detecting a second set of objects in the second image data; calculating a common coordinate system for the first set of objects and the second set of objects; converting locations of the objects in the first set and/or the second set into the common coordinate system; targeting of said objects in the first set and the second set based on positions of the objects in the sets; targeting of said objects in the first set and the second set that are located outside an area of overlap of the first image and the second image; and outputting data indicating zero or more of said objects in the first set and the second set as potential changed objects, wherein the step of eliminating zero or more objects in the first set and the second set based on positions of the objects in the sets includes: calculating a distance from at least one object in the first set and a nearest neighbour of that object in the second set; calculating a distance from at least one object in the second set and a nearest neighbour of that object in the first set; eliminating a said object in the first set if the distance between that object and the nearest neighbour of the object meets one or more culling criteria; and eliminating a said object in the second set if the distance between that object and the nearest neighbour of the object meets the culling criteria.