Digital USGS topoquads for the State of Georgia are currently being added to the library catalog therefore coverage is focused on the Metro Atlanta region. Also, in prodcution is a Georgia Topographic Index Layer for Google Earth.
A 7 1/2 minute quadrangle or "quad" as they are often reffered to is a map representing an area of 7 1/2 minutes of latitude by 7 1/2 minutes of longitude. Every 7 1/2 minute USGS quad is named for a pominent feature found on the map (e.g. a city) and all of them have the SuDocs number stem of I 19.81:.
The library's collection of quads are arranged first by state, and then alphabetically by quad name. For example, the quad for Houma, Louisiana (published in 1998) can be found in the drawer labeled Louisiana: Golden Meadow - Kurthwood. We have assigned that quad the SuDocs number I 19:81:LA/HOUMA/998.
Note: Not all of the quads in these map cases have been assigned a SuDocs number but they are all filed by the same alphabetical by state and then by quad name rule.
7 1/2 minute quadrangles are called topographic maps or "relief maps" because they show topography or relief (elevations and depressions of the landscape). Most of the 7 1/2 minute quadrangles in the collection use countour lines to show relief.
Note: All points on a contour are the same elevation (think of a contour as an imaginary line on the ground that takes any path necessary to maintain a constant elevation). Each contour on a quad represents a different elevation, and the contour interval is the difference in elevation between adjacent contours. On steep slopes, the contours are spaced more closely than on gentle slopes (see figures below).
Elevation is shown by a 3D effect of the top while the same area is drawn as a map with contours showing elevation on the bottom. Note that the contours are close together where the slope is steep, and further apart where the slope is more gentle.
Other symbols used on USGS quadrangles may be identified by using the guide to Topographic Maps Symbols offered by the USGS.
Sometimes, the city or area of coverage you're interested in does not have a named quad. For example, if you were interested in looking at the quad for Gulfport, Florida, you would not find it by looking for it under SuDocs number I 19.81:FL/GULFPORT in the drawer labeled FLORIDA: Graceville - Keaton Beach because there is no quad with that name or SuDocs number.
To find the 7 1/2 minute quad that contains Gulfport, Florida, you would have to refer to FLORIDA: Index to topographic and other MAP COVERAGE located in the grey metal box on top of the map cases.
Note: Because 7 1/2 minute quadrangles have a map scale of 1:24,000, you would locate Gulfport on the primary map series, 1:24,000 scale index map.
Note: A scale of 1:24,000 means that 1 unit on the quad represents 24,000 units in the real world. For example, one inch on the map represents 24,000 inches in the real world, or expressed another way, one mile (which is also 63,360 inches) in the real world is represented by around 2 1/2 inches on the map.
Next, you would locate Gulfport, Florida, on that index map. ** Gulfport is on the west coast of Florida, on the west coast of Florida, south of St. Petersberg, and on the south end of Pinellas County which is the peninsula just to the west of Tampa Bay.
The index map hs the names of the quads (printed in brown and at a 45 ° angle). Once you've found Gulfport, you would note that the name of the quad that contains it is Pass-a-Grille Beach. You would then find that quad (with SuDocs number I 19.81:FL/PASS-A-GRILLE BEACH/994) in the drawer labled FLORIDA: Orlando - Putnam Hall.
Digital USGS topoquads for the State of Georgia are available either through the library's EUCLID catalog or you can dowload the Georgia Topographic Index Layer for Google Earth (available soon).
USGS Cartographic Data
US Geological Survey manages and distributes many kinds of cartographic data, including GeoPDF data and the new “DigitalMaps – Beta.” Information for most USGS cartographic data is available at http://www.usgs.gov/faq/list_faq_by_category/get_answer.asp?id=317.
About GeoPDF data files
PDF (Portable Document Format) digital files are now available for US Geological Survey topographic quadrangle maps. Each file is essentially a scan of a topographic map with the added feature of being georegistered. The files can be used as a PDF file, enabling users to view topo maps onscreen.
The GeoPDF format is an extension to Adobe's PDF 1.3 and higher versions enabling GIS functionality within standard PDF files. This format is designed for the efficient distribution and communication of rich spatial data to anyone who needs to view, review, verify, update, or print it. Because GeoPDF files are highly compressed and encapsulated, they are smaller, faster, and easier to transmit than GIS data sets, without the overhead associated with typical GIS spatial data sets (or the management of database tables, external links, and dependencies). Using the GeoPDF format, publishers of spatial data can select the specific spatial data they want recipients to see and can publish GIS source files into a single GeoPDF file.
GeoPDF files are not a replacement for native GIS formats. GIS professionals still need the original files for editing or updating spatial data. GeoPDF files enable non-GIS professionals, field technicians, business executives, and their colleagues to utilize rich spatial information. Users can view and print GeoPDF files with the free and ubiquitous Adobe Reader ,and they can do more with the data using a free plug-in called TerraGo Desktop. Users do not have to install this plug-in to view GeoPDF files.
About the data available in GeoPDF format
The initial set of GeoPDFs was produced by the US Army Corp of Engineers Topographic Engineering Center using existing USGS Digital Raster Graphics (DRGs) (scanned at 250 pixels per inch (ppi)) as the source. The file names reflect the DRG from which they were derived. A USGS project is underway to upgrade all 250 ppi DRGs with higher resolution (400 ppi to 508 ppi) files, which can produce high quality print-on-demand products. Depending on the detail and complexity of the source map, individual GeoPDF file sizes vary from 3 to 30 megabytes, with the majority being in 10-17 megabyte range. In most cases the GeoPDFs reflect the latest version of the published map; those that don't are continually being replaced with higher resolution files created from 508 ppi scans of the paper map.