This is especially important when doing the kinds of things VO astronomers want to do with access to images from many sources, nearly all of which rely on a correct coordinate system in each image being examined. The registration of images for stacking and mosaics, identification of objects in a crowded field given a reference catalog, crossmatching object detection catalogs with reference catalogs (see Chapter 14), and matching detections in an image of one bandpass with the same object in another, are just a few examples. The NVO Science Steering Committee recognized both the current state of coordinate systems in images available through VO services and the importance of this fundamental property of the data to enable good science when it rated the development of a service to correct an image WCS as a priority.

We describe the WCSFixer web-application (http://iraf-nvo.noao.edu/wcsfixer) as one means by which astronomers can improve (or derive) the WCS solution for an image, its implementation and the constraints imposed by that implementation.