Sanborn was an insurance adjacent company that made detailed paper maps of many towns in the US in the late 1800s and early 1900s.

This project is scanning those maps and adding the information that modern mapping software needs to show them alongside other geographic information. Where the old paper maps overlap, they work to cleanly stitch them together rather than showing the edges of the paper.

Later: The project is sort of for people looking for the old maps, so I can see why the introduction doesn't spend a life of time explaining them.