digger vs sapper

digger

noun
  • A tool for digging. 

  • A spade (playing card). 

  • An Australian soldier. 

  • A large piece of machinery that digs holes or trenches. 

  • One who digs. 

sapper

noun
  • An officer or private of the Royal Engineers. 

  • Historical term for a combat engineer that is still used in some armies, in other words an engineer or a soldier engaged in attacking, destroying, and circumventing or building fortifications, bridges, and roads; a military engineer active in a combat zone. 

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