SAR is a statistical numbers game. Each search method provides some degree of certainty of coverage. Aircraft can't see through trees and thermographic camera

SAR is a statistical numbers game. Each search method provides some degree of certainty of coverage. Aircraft can't see through trees and thermographic cameras have a tough time seeing hypothermic or dead people. Ground search is still necessary even after a helicopter or fixed wing aircraft have gone over a search zone. A swarm of drones would allow a rapid search of an area with similar coverage statistics as a manned ground search. But a drone going 40 km/h could miss subtle clues like footprints or freshly broken branches.