angular mil vs imperial system

angular mil

noun
  • A unit of angular measurement equal to 1/6400 of a complete circle. At 1000 metres one mil subtends about one metre (~0,98 m). Also 1/6000 and 1/6300 are used in other countries. 

imperial system

noun
  • A system of measurement in use in the United Kingdom and other Commonwealth countries, now mostly superseded by the metric system; similar to the system currently used in the United States; consisting of units such as the inch, the mile and the pound (of weight). 

How often have the words angular mil and imperial system occurred in a corpus of books? (source: Google Ngram Viewer )