Scanned Topo Maps (Digital Raster Graphics)

In The National Map viewer's table of contents, select the Overlays tab, click on the Other Featured Data panel, and then select the Scanned Topo Map
Information on the historical topographic map collection is available at The National Map: Historical Topographic Map Collection site.
The phrase digital raster graphic, or DRG, can mean several things.  It is a generic term used to mean almost any scanned image of a paper map.