First and foremost, USGS topo maps. CalTopo's usgs layer is composed entirely of 7.5'x7.5' quads tiled together - somewhere in the neighborhood of 120,000 of them, although I don't know the exact number. The USGS has 7.5' maps available from three different projects:
- 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.
CalTopo's topo layer is built from historic topographic maps, with US Topos filling in where historic topos were unavailable or improperly georeferenced. The following states have not been scanned yet and are based off DRGs: WY, SD, LA, MS, FL, RI, MA, VT, NH. California topos are based on a separate scanning project from Cal-Atlas.
The US Forest Service produces its own set of 7.5' quads with updated road and trail information, typically without vegetation shading; these are called Primary Base Series maps. Those of you familiar with MyTopo's recently-restricted map layer may recognize that MyTopo appears to have used these in place of USGS quads where available, with green shading added on top. Although CalTopo only has coverage for California, expansion to other western states is on the roadmap.
The USGS also supplies 1/3 arc-second elevation data (approx. 30ft) as part of the National Elevation Dataset, and this is used to produce the Shaded Relief and 40 foot contour layers. Like the DRGs, this is only available if you ship a drive to the USGS. Relief shading was done with GDAL, specifically the gdaldem utility at 3x vertical exaggeration, and then tiled. The 40' contour layer was made using gdal_contour to produce contour vectors from the NED and mapnik to render those vectors into images. Both layers required a healthy dose of judgement calls; I tried to make decisions that would render best on top of a wide variety of backrounds, but the topo lines may be too thin on some layers and too thick on others.
The NAIP Aerial layer was built from the USDA's National Agriculture Imagery Program. Google has higher-resolution imagery, but it seems to always have ugly seams right where you don't want them, and they photographed much of the Sierra when it was covered in snow, which makes the imagery useless. I also wanted a cacheable version that we could put on a drive and take with us for SAR. The existing layer was built from the .SID files on Cal-Atlas, but I plan to expand coverage using the image layer from US Topos at some point in the future.
The 1900 layer was built from scanned maps available on several sites linked to from http://www.sdc.ucsb.edu/holdings/caltopo.html. All maps are from 1885-1915. Datums were still in shift at this point so I had to make some educated guesses about the map's datum based on publication date; the result is that some maps are offset a bit. The linked site has much better historical map coverage than the USGS, but I plan to expand coverage beyond California using the USGS's historical topographic map collection.
The Visitor map layer was pieced together from digital copies of USFS, NPS and BLM visitor maps. Coverage is USFS/NPS in California, and national parks/national monuments in the western US. I'd like to grow this layer in the Western US, especially the southwest, but it's painstaking work. Each map requires georeferencing 3-4 pixel locations against real-world coordinates, and when two maps overlap, you need some surgical use of the eraser in Gimp or Photoshop.
The OpenCycleMap layer is provided courtesy of the OpenCycleMap project; I have nothing to do with it. It's the best vector-based climbing/hiking/biking map I've seen; it rocks in some locations, while trail coverage is sparse in others.
Terraserver images are provided by terraserver.com. They're lower-quality and slower than the CalTopo layer, but cover Alaska.
MODIS maps are provided by the USFS Active Fire Mapping Program. They're a little slow to load.