A cartography question: you open an atlas and usually at the beginning of it there are some world maps showing various thematics. Let's say the world map uses Robinson projection and it says it has a scale of 1 : 120,000,000.
How is this scale determined? Are there any standard practices (measuring scale ratio of 10° longitude on equator or something similar)? Or is this an imprecise science left to each cartographer to figure out?