They don't use green screen. Before the game with the field empty they do a pan and scan of the entire field from every camera. That gives the system a baseline for change detection that works regardless of team colors.
Re:Virtual lane graphics for swimming with CSS3
by Zimahl on Monday March 04, 2013 @01:25PM
(#5319635)
By 'green-screen' I meant that they take a small snapshot of the field and use those colors as a replacement. A scan before the game would do them very little good, especially in open-air stadiums where game conditions can change immediately.
Well apparently it does them a lot of good, because that is what they do. Check out the Wikipedia article on First and Ten I posted further up in the thread.
College and NFL football typically has 3 to 4 stationed cameras up at ~30 degree angle. These cameras are bolted down and do not move from their positions. They are able to transmit their zoom, angle, and pitch very precisely which allows the production team to know exactly where they are pointing at all time. During the transmission they can choose the exact location for the first down markers, etc., and all the high-angle cameras will add it in. To do the overlay they choose a swatch on the field to use as a green-screen. They only trouble that arises is when a teams uniforms is very close to the field color.