picket fence vs stockade

picket fence

noun
  • A simple fence made from wooden pickets, connected by horizontal bars. 

  • A series of spaced perpendicular striations that occasionally occur in aurorae. 

stockade

noun
  • an enclosure protected by a wall of wooden posts 

  • a military prison 

verb
  • To enclose in a stockade. 

How often have the words picket fence and stockade occurred in a corpus of books? (source: Google Ngram Viewer )