For the lists of objects identified with the data and algorithms of Paper I of this series, photometric and astrometric calibrations were performed according to the algorithms of Paper II. The software system then assembled these results into the Guide Star Catalog (GSC) with the use of a set of (primarily noninteractive) image-processing and calibration programs interconnected by a set of pipeline files and supported by databases organized on a plate-by-plate basis. A set of utility programs was also provided to support quality control and to correct operational problems. Object names are of the form GSC rrrrr nnnnn, where the first field specifies an internal region number and the second is an ordinal within it. For objects cataloged from more than one photographic plate, an entry is made for each image. Each object is given a unique name; a matching algorithm identifies data from multiple images of the same object so that all entries of the same object are assigned the same name. A number of details concerning the structure and integrity of the GSC are also described. These include the database for compiling statistics of objects with multiple entries, as well as the provisions for assigning names, for cataloging objects lying in the plate overlap regions, for rapidly indexing positions against regions, and for recovering the original plate measurements (x,y, and photometric parameter). The GSC is stored as 9537 files, each containing approximately 2000 objects. The export form of these files, as well as of the auxiliary files that describe the contents and use of the GSC, is the FITS table format. As a tool to support user evaluation of the completeness of the GSC, separate count statistics for stellar and nonstellar objects are provided on a plate-by-plate basis.
