necropolis vs urnfield

necropolis

noun
  • An ancient site used for burying the dead, particularly if consisting of elaborate grave monuments. 

  • A cemetery; especially a large one in or near a city. 

urnfield

noun
  • Ground used as a cemetery in Bronze Age Europe, in which the ashs of cremations were buried in cinerary urns. 

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