Historical Topographic Maps

As of late 2014, more than 95% of the historical USGS topographic maps - almost 179,000 map sheets - have been scanned and published. An estimated additional 4% are in process and are expected to be published in 2015 and 2016.
Almost 180,000 USGS historical topographic maps are available free of charge as PDF documents. This set of files is referred to as the Historical Topographic Map Collection (HTMC). 
USGS does plan to publish GeoTIFF versions of the Historical Topographic Map Collection, perhaps starting in 2015, though exact schedules have not yet been set. For information on converting GeoPDF to GeoTIFF, see
Yes, these are different maps, though the differences are often relatively minor. For a particular cell and scale, most maps have only one original base compilation, but most have later revisions and reprints.
Map sheets were initially published at three primary scales (1:62,500 scale; 1:125,000 scale; and 1:250,000 scale) with contour intervals of 10, 20, 50, 100, or 200 feet depending on the terrain.
There is no single document that describes all symbols used on all USGS topographic maps.
Metadata, as described by the Federal Geographic Data Committee, is "the background information which describes the content, quality, condition, and other appropriate characteristics of the data.
Yes. FGDC-compliant metadata will accompany each map.
The USGS completed the primary topographic map series of the conterminous United States at 1:24,000 in 1992. Revisions were made until 2006. Alaska, Hawaii, Puerto Rico, and Trust Territories were completed earlier and were done at other scales.
The goals of the Historical Topographic Map Collection (HTMC) project are to find, catalog, and georeference all USGS topographic quadrangle maps, for the purposes of preserving the USGS map archive and making that archive available to scientists, hi