Whenever there is a brush fire or forest fire that covers a large amount of territory, how do they determine how many acres have already been burned and how do they determine how much of the fire is actually contained at any given point?

I live in Arizona where we have had several large brush fires over the years and the news anchors will say something like “The fire has charred 463 acres and is 34 percent contained at this point.”

How can they accurately measure the acreage and how do they figure out the containment percentage? Would it all be guessing by the professionals or do they have an actual formula they employ to find their answers? Anyone have any facts or theories about how firefighters find these answers?