Historical Topographic Maps

Almost 179,000 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). 
The following table summarizes the map series included in 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.
There is no single document that describes all symbols used on all USGS 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.
The goals of the Historical Topographic Map Collection (HTMC) project are to find, catalog, and georeference all USGS topographic quadrangle maps. The overall purpose is to preserve the map archive for scientists, historians, and the public.
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.