[dmm]

I worked for the USGS for a few years and learned a lot about this.

The old topo maps were compiled, drawn, and edited by hand, from primary sources. Look at an old topo and notice the different symbols for barns and houses, outline vs filled squares. They were able to mark those because a usgs employee went out in the field and checked each building. The result of this process were extremely accurate and detailed maps but it was very expensive. It took hundreds of person-hours for each map.

The new topos are produced from remote sensing data, mainly imagery and lidar, and are highly automated. They are worse in most ways(but quickly improving!) than the old maps but take much, much less person time, less than 10 minutes per map. This time is mostly spent verifying/cleaning the source data.