CalTopo map coverage for the contiguous (i.e. lower 48) United States is almost done. Given that the map layers are about to be refreshed, this seems like a good opportunity to write about what is currently available, and where the data comes from.
- Digital Raster Graphics. Although all their map scans are technically DRGs, the USGS uses this to refer to 250ppi maps scanned for Microsoft’s Terraserver project. These have the advantage of normalized colors (same shade of green everywhere) only occupying 238GB of drive space. Downside is the low scan quality, which is evident in Terraserver’s map tiles. You can’t get this online anywhere; you have to ship a drive to the USGS.
- Historic Topographic Maps. The USGS has been re-scanning every map they can find (not every map ever made, although coverage is quite good) at 600ppi and making the result available as georeferenced PDFs. The scan quality is amazing, but the colors and even the intensity/darkness vary heavily from scan to scan. These are called historical topographic maps, which is a bit confusing at first as it includes maps printed as recently as 10 years go. More Information.
- US Topo Maps. The USGS has replaced their traditional printed maps with a product called “US Topo”, the successor to their “Digital Map – Beta” line. You can still order printed copies of US Topos, but the source map is built electronically by compositing shapes and place names on top of vegetation coloring and aerial imagery. These are available as multi-layer PDFs; you can pick individual layers with the right tool, but many PDF viewers show you all of the layers composited together (with aerial imagery covering up the vegetation layer). Download US Topos.