observation post vs vantage point

observation post

noun
  • a lookout or vantage point 

vantage point

noun
  • A place or position affording a good view. 

  • A point of view; perspective; outlook; standpoint. 

  • A camera angle. 

How often have the words observation post and vantage point occurred in a corpus of books? (source: Google Ngram Viewer )