Assuming you already know what continent somebody is on, 20 circles of 200km radius (120 miles) should cover most of the major population centers. If you live

Assuming you already know what continent somebody is on, 20 circles of 200km radius (120 miles) should cover most of the major population centers. If you live out in Nebraska or the middle of the Sahara this attack is easy to defend against, but humans tend to clump up.