One of the things we noticed when mapping Noda Bi-Yehuda is that the map for Vol. I looks different from the map of Vol. II, and that his sphere of correspondence seems to shrink over time. Take a look at what I wrote in the post about this.
We now have another way to visualize this: distinct heatmaps for the two volumes. You really see how Vol II covers a far more compact area, especially relative to the overall number of responsa. Take a look:
It is only appropriate to point out that some of this fits with the location of the author. Rabbi Ezekiel Landau was from the province of Galicia, and his first rabbinic posts were in that area. This would have given him a network of questioners and halachic discussants. But his later career was in Prague and made him the pre-eminent authority in central Europe.
Thank you. We should have pointed this out. However, this accounts for some, but very little, of the changes we see. He arrived in Prague in 1755 and NBY I was published in 1776. Of 33 dated teshuvot in Vol I, only 2 are from before the move to Prague.