Measurement.define(:'th.') do |unit|
  unit.alias :th, :thou
  unit.convert_to(:mi) { |value| value / 63_360_000.0 }
  unit.convert_to(:fur) { |value| value / 7_920_000.0 }
  unit.convert_to(:ch) { |value| value / 792_000.0 }