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All-sky photographic surveys made with the large Schmidt telescopes have long been a major resource for research support; within a few years the second generation surveys now in progress with the UK and the Oschin (Palomar) Schmidts will generalize this coverage to at least two epochs and two passbands. These surveys are summarized, with some attention to their potential applications in research and in telescope operations. The quantitative use of these plate collections depends on their digitization. The properties, accomplishments, and plans of the five modern high-speed measuring machines currently devoted to this work (APM, Cambridge; SuperCosmos, ROE; GAMMA, ST~ScI; APS, Minnesota; and PPM, Flagstaff) are described with special attention to the catalog and the scan data-products.
As the scan archives corresponding to these plate collections occupy terabytes, some form of data compression is an essential element of their use and distribution. The most extreme compression is not truly a compression but the construction of secondary data products: object catalogs. While such catalogs in some form are invariably useful, many programs are best pursued with the image data. Two compression modes, both based primarily on controlled loss of sky-data, are used for this: (a) thresholding, wherein only image sections containing objects are saved, and (b) full-plate compression, wherein an algorithm such as the H-transform provides the basis for isolating and deleting some small fraction of the sky-noise. Both procedures easily produce compressions of order 10$\times$.
The current methods of distributing the data, ie, by computer copying, printing on CD ROMS, and network access are discussed; and the tradeoffs among them are compared. Finally, a few remarks are made regarding the future of plate scanning, the options for specialization among the measuring community, the future of photography, and possible interactions with the forthcoming all-electronic sky-survey programs.
