File: MAPINDEX.TXT Date: September 5, 1995 ************************************************************************** MAP REFERENCE INFORMATION FOR DRG PRODUCTS Scanned raster images of Map Indexes are included on this CD-ROM as a convenient cross reference between file and quadrangle names. When displayed, the images can also be used to relate a quadrangle to a particular geographic area. The 500 dpi, PackBits compressed TIFF images of the Indexes average 1 Mb in size and are stored in the \INDEX directory. GEOREFERENCING DATA ARE NOT AVAILABLE FOR THESE IMAGES. Each Index contains a 1- by 1-degree secondary cell composed of a grid of sixty-four 7.5-minute quadrangles (primary cells) registered to a small- scale planimetric map. Individual quadrangles are identified by a two- character alphanumeric Map Reference Number. Quadrangle names appear opposite the Map Reference Number. The viewing software included on this disc can be used to display these Map Index images. When properly loaded and displayed, the user can peruse the gridded planimetric map to obtain a geographic overview of the entire area comprising the 1-degree cell. Planimetric detail includes features such as: Major civil subdivisions Transportation features Hydrographic features Villages, towns, and cities ************************************************************************** DATA SET NAME (DSN) CONVENTION APPLIED TO MAP INDEX IMAGE FILES Each secondary cell is referenced by a unique 5-digit numeric code identifying the southeast corner of the cell by geographic coordinates. In the displayed image, this code is centered directly above the 1-degree cell. A list of quadrangle names with their respective Map Index Numbers is located opposite the 1-degree cell. By combining the 5-digit geographic code with the two-character Map Index Number, the user can determine the location, DSN, and quad name of all DRG's within the cell. For additional information about the DSN convention for DRG's, refer to DRG_NAME.TXT, accessible from the \DOCUMENT directory. A generic example of the DSN convention applied to the Map Index file(s) included on this disc is shown below: Example: MO37090.TIF Where: MO = Two-character State abbreviation 37 = Degrees Latitude of the 1- by 1-degree secondary cell 090 = Degrees Longitude of the 1- by 1- degree secondary cell TIF = Tagged Image File Format ************************************************************************** MULTI-STATE 1-DEGREE COVERAGES Individual secondary cells may cover portions of several States. If areas of more than one State are contained within a cell, Map Index images of each additional applicable State are included on the disc as separate Index files. Example: MO40091.TIF = Index file from the Missouri Map Index IA40091.TIF = Index file from the Iowa Map Index IL40091.TIF = Index file from the Illinois Map Index In this generic example, scanned pages from the Missouri, Iowa, and Illinois Map Indexes would be included on a single disc. ************************************************************************** VARIATIONS TO THE MAP INDEX 1-DEGREE BLOCKING SCHEME Individual Indexes of the conterminous United States, Hawaii, Puerto Rico and the Virgin Islands will normally cover a 1- by 1-degree area. However, oversize, undersize, and offset cell configurations occur along coastal margins and islands. Indexes of the Pacific Islands contain either 1 or 2 cells formatted in 1- by 1-degree blocks, although some cells are optionally referenced to the 30-minute grid corner instead of the normal 1-degree grid corner. Individual quadrangles within these secondary cells may be oversize, undersize, or shifted from the normal 7.5-minute grid to facilitate coverage along coastal margins and islands. In Alaska, the number of 1- by 2-degree cells, south of 59 degrees latitude, and 1- by 3-degree cells, north of 59 degrees latitude, will vary on each Index. Oversize, undersize, and offset cell variations occur along coastal margins and islands. ************************************************************************** PRECONFIGURED MAP INDEX FILE VIEWS The Map Index files included on this disc have been configured for convenient display using ArcView 1.0 software. These Index files are stored in the \INDEX directory and are accompanied by associated ArcView Views of the same name with a .AV extension. Selected Map Index images, preconfigured in two adjacent windows, can be retrieved and displayed using the FILE - OPEN option from the ArcView Menu. One window displays the 1- by 1-degree secondary cell containing a planimetric map with sixty-four 7.5-minute grids superimposed. The other window contains a sorted list of quadrangle names prefaced by their respective Map Reference Numbers. **************************************************************************