Vast majority are currently from the USGS, but this is going to wildly shift and diversify soon as I've been working to bring a wider variety of sources. The next wave is coming mainly from public library systems from all across the globe (my background is in search so I literally am running a map crawler)

I stand on the shoulders of these giants that have done amazing work to digitize the paper maps and I mainly am hoping to just aid in the ease of discoverability and exploration of these assets