football field vs rugby union

football field

noun
  • A playing field on which the game of Association football is played. 

  • An informal unit of measure of area, equal to the area of an American football field (57,600 square feet or 1.32 acres). 

  • A playing field on which the game of American football is played. 

rugby union

noun
  • A form of rugby football, historically amateur, having 15 players per side; points are scored for a try, penalty, drop kick or conversion. 

How often have the words football field and rugby union occurred in a corpus of books? (source: Google Ngram Viewer )