In 2015 the National Geologic Map Database project mass-converted all GeoPDF HTMC maps to GeoTIFF and several other image formats.
USGS Digital Raster Graphics (DRGs) were created in the period 1995-2000. They were 250 dots-per-inch scanned images of what were then the most current editions of topographic maps (approximately 58,000 maps).
Superseded US Topo maps will remain available for download, but will not become part of the Historic Topographic Map Collection (HTMC).