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The descriptions cover about 10,000 items, roughly one-third of the described maps and plans in the total map collection. You may, however, find it useful to consider the particular types of information or elements that are consistently presented in the descriptions. Major Regions For Indexing Purposes 1975 . Index to Numbering of Map Sheets, National Topographic Series 1975 for N.T.S. Divisions. Note that individual items are identified by a specific creation or publication year.