We did this in 2018 and in 2019 (like in 2019, I will probably do either daily or round-by-round updates, rather than game-by-game this season). The more detailed explanation is in the 2018 thread, but the general principle is that each county in the country is assigned to the 68 NCAA tournament teams by geographic proximity (distance from home arena to geographic center of the county) and then the map consolidates as the games are played (winning team takes losing team's land). Here's the starting map for 2021.