For those of us who hate drawing circles in the dirt there is a clean and snappy solution: the "Rond de Petanque 50"
Simply pull out your RDP from your pocket, and snap open each of the segments to form a perfect and legal 50 cm circle. Place on the terrain - and play!
No, I don't sell the thing, I saw it on EBAY (france) - just look under the search term: PETANQUE
I can't wait until someone invents a 4m x 14m terrain which unfolds from a briefcase!