There are many instances where it is necessary to know the diameter of a cancellation in order to distinguish it from similar ones. If the cancel is entirely visible and was clearly struck then measurement is straightforward, but often neither is true, and estimating may involve significant guesswork. If you have access to image manipulation software which allows the use of layers (such as Photoshop, or the free programme GIMP) you may find that Ray Harris circles can give you an accurate answer, even with cancels in which less than half the circle is visible.

The circles are a series of accurately drawn, at 400dpi, on a transparent background which you can place as a layer above a scan of the cancel in question (also at 400dpi), and move until you see a match between the arc of the cancel edge and the curvature of the circles on the transparency. The result is reliable to within 05mm.

For consistency, make measurements from the centre of the cancel boundary line rather than the inner or outer, and bear in mind that some round cancels not perfectly cicular

The Ray Harris Circles transparency is available to download as a .png file.