Napa County authorities have a short and succinct motto for those who need to know whether to evacuate or keep watch during future emergencies: Know Your Zone. An evacuation mapping system rolled out by the Sheriffâs Office divides the county into more than 250 numbered zones that can be targeted for alerts about wildfires and other threats to life or property. Operated by the San Francisco software company Zonehaven and previously adopted by Sonoma, Marin, and other California counties, the zone-based alert system is intended to more rapidly and precisely target public safety announcements during disasters, and reduce confusion about which areas are under the most immediate threat.