From the topoView web site description:
TopoView highlights one of the USGS's most important and useful products, the topographic map. In 1879, the USGS began to map the Nation's topography. This mapping was done at different levels of detail, in order to support various land use and other purposes. As the years passed, the USGS produced new map versions of each area. The most current maps are available from The National Map. TopoView shows the many and varied older maps of each area, and so is useful for historical purposes—for example, the names of some natural and cultural features have changed over time, and the 'old' names can be found on these historical topographic maps.
This interface was created by the National Geologic Map Database project (NGMDB), in support of topographic mapping program managed by the National Geospatial Program (NGP). Geologic mapping and topographic mapping at the USGS have a long tradition together (see 1888 report). The NGMDB project is proud to assist the NGP in bringing these maps to the Web.
