Is there an explanation for the measurement system used in aviation?
To quote from the Wikipedia article on the subject:


  Although the unit knot does not fit within the SI system, its
  retention for nautical and aviation use is important because standard
  nautical charts are on the Mercator projection and the scale varies
  with latitude. On a chart of the North Atlantic, the scale varies by a
  factor of two from Florida to Greenland. A single graphic scale, of
  the sort on many maps, would therefore be useless on such a chart.
  Since the length of a nautical mile, for practical purposes, is
  equivalent to about a minute of latitude, a distance in nautical miles
  on a chart can easily be measured by using dividers and the latitude
  scales on the sides of the chart. Recent British Admiralty charts have
  a latitude scale down the middle to make this even easier.[8]
  
  
  e.g. BA Chart 73, Puerto de Huelva and Approaches, 2002