At these levels we may envisage the archive as a library which lends out books but is loath to rip out individual pages. A level 4 archive recognizes the intrinsic malleability of digital data and can extract elements from the various archive files for processing prior to delivery to the user. The element that distinguishes SkyView as a level 4 archive is that it generates its products for the user dynamically. When a user requests an image the system determines the parameters of the request and extracts and manipulates data from the existing all-sky surveys. Then it creates an output product to the user specifications.
Several things are key to making it possible to have a fourth-level archive.
First, there must be some agreement among the community of the scope and type of data manipulations possible. If there is no way to predict the kinds of manipulations that would be useful the system may have very limited appeal. SkyView deals with clearly defined geometric transforms. While there are a number of coordinate projections and coordinate systems this number is manageable. Similarly, it is very important that there be some way of presenting the data to the user in a fashion that he or she can be expected to understand.