You see, place (a place being the sort of thing that is described by a geographic identifier, or toponym) is what holds the Imperiia project together. It is the element that allows us to analyze pieces of information that come from different sources and talk about different things as a coherent set.
So basically the constant to evaluate the same maps was that they has the same administrative geography or vertical relationship between places: Provineces / states / departments, counties, cities, villages.