Is Ground Direction Finding commonly used? I've recently come across the concept of Ground Direction Finding. My (limited) understanding of it is that it is a system that allows you to see where a VHF transmission is coming from. I gather it was used in the Earhart-era of aviation for navigating, and could be a rudimentary air-traffic control system where there is no radar. 

I think it's still used these days in boats but what about aviation? And is my understanding of it essentially correct?
Yes, it is commonly used, especially in large complex airspaces. 

If two VHF direction finders are installed at sufficient distance from each other, they allow the ATC system to triangulate the position of a VHF transmission. This position is then indicated on the air taffic situation display, which helps the air traffic controller to quickly correlate the transmission to the transmitting aircraft, often before the callsign has been transmitted. The controller still has to crosscheck the callsigns, but it helps to reduces workload.

Example of a VHF direction finder