1. An image-based measurement system, comprising:— active imaging means ( 101 , 102 , 103 ) operable to provide an active image of a subject of measurement, showing the subject when active or in use, reference image means ( 104 , 105 ) operable to provide a reference image of the subject of measurement, the reference image being of an image type different from the active image, image combining means ( 106 ) operable to combine the reference image and the active image, matched in scale and keyed together, and image measurement means ( 107 ) operable to effect measurement of the active image relative to the reference image, to provide measurements of dimensions of components and/or spacings between components of the subject of measurement, when active or in use.