“We’ll just have to think our way back, step by step.” That’s what the Dutch geodesist Roelof Nicolai said in 2014 in a Strange Maps article . Nicolai had just spent his PhD years discovering that portolan charts — those gorgeous, wind-rose-covered coastline maps that suddenly appeared in Mediterranean ports in the second half of the 13th century — were geometrically too accurate to have been made the way everyone assumed. Something else was going on.

Twelve years on, we might have an answer. Tome Marelić, an assistant professor of geography at the University of Zadar in Croatia (and the current vice chair of the Working Group on the History of Cartography within the International Cartographic Association), has spent years doing exactly the step-by-step thinking Nicolai prescribed in 2014. Read together, his scientific articles amount to the most detailed forensic examination portolan charts have ever received.…