Thanks for the kind words.
The Pennsylvania maps were originally published anywhere from 1891 to 1948. Each map's file name contains the publication date of the first edition of the map. For example, if you position your mouse cursor over "Philadelphia" in the Pennsylvania grid, you'll see a line ending with "Philadelphia1898.jpg" in your browser's status bar.
After you look at enough of these maps, you can roughly estimate when one was originally pubished by checking a few details. For instance, most of them started out as engraved plates, in the same fashion as banknotes. Water features, like rivers and lakes, are made up of parallel blue lines. But engraving isn't cost-efficient, so after 1940, engraved maps were out, and water features are solid blue. If you see the words "War Department" printed at the top of a map, you can be sure it was produced during World War One or Two. Highway route numbers are another tipoff. At first, there aren't any. Then, in the 1920s, there are a few--so few that the state roads are labeled with something like "Pa. Route No. 123". It isn't until the 1930s that numbered highway route shields replace the text. And while road names are few and far between, railroads are carefully labeled. Even branches are named. I suspect that where lines and yards indicate multiple tracks, even the number of tracks is correct.
