What does it mean when a wildfire is 40% contained?

I’ve been hearing about the Angora fire near Lake Tahoe on the radio news on the way home from work. They were saying yesterday that the fire is 40% contained. My question is, what does “40% contained” mean, exactly?

I can think of two possibilities:

  1. 40% of the perimeter of the fire is reasonably secured against further spreading

  2. There is a 40% chance that the fire will not spread further than it already has

Anybody know?

I’m pretty sure that your first choice is the correct one, but I don’t have any cites to back that up.

#1 is correct.