This is great, John! You might be interested to know that the NYPL Digital Experience Group is actually working with the Maps Division as we speak on a new mapping tool that will allow you to make these sorts of comparisons directly, that is by literally layering the two maps together through a process called "goerectification." Essentially, you select several "ground control points" — places on the historical map for which you have some sort of contemporary data (i.e. Waverly and Sixth, which is in the same place today as it was in 1846). We then add latitude/longitude data and repeat the whole process with several more locations on the map. Now the computer has what it needs to reorient the historic map to overlay the contemporary one. Suddenly the old map is no longer just a visual artifact, but a usable, interactive object which we can navigate, label, annotate and connect to other resources. Imagine!

We're building this tool not in Google Maps but in an open source equivalent called Open Layers. Stay tuned for more news this summer!