CityLab maps the geography of mass shootings in America since 1982, “both geographically and by number. ‘Mass shooting’ is defined as an incident during which four or more people are killed during a single attack in a public place, excluding the shooter. This is one of the more conservative counts—the data does not include shootings that took place during conventional crimes, like armed theft or gang violence.”

See also Vox’s collection of maps and infographics and Maps Mania’s roundup of gun violence maps.