North American football vs football field

North American football

noun
  • Collectively, Canadian football and American football. 

football field

noun
  • 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 Association football is played. 

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

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