The present invention provides an image-based measurement system or method wherein there is provided (101) an active image of a subject of measurement, showing the subject when active or in use, and there is provided (104, 105) a reference image of the subject of measurement, the reference image being of an image type different from the active image, and wherein the reference image and the active image are combined (106), matched in scale and keyed together, and measurement (107) of the active image relative to the reference image is effected to provide measurements of dimensions of components and/or spacings between components of the subject of measurement, when active or in use. The invention further provides an image-based measurement system or method in which active images of a subject of measurement can be used to derive a reference image, for example a CAD design image.