People are always surprised when I tell them about Zack Walker, Airbnb’s in-house cartographer, who works on mapping the world. They say, “the world is already mapped! There’s Google Maps, Foursquare, Yelp, Garmin, National Geographic, Zheng He, Isabella Bird, Lewis & Clark & Sacagawea, Leif Erikson, the Planet Earth series narrated by David Attenborough…” Which …