Measurement.define(:'ft²') do |unit|
  unit.alias :ft2, :'ft*ft', :'sq ft', :'square foot', :'square feet'
  unit.convert_to(:mi2) { |value| value / 27_878_400.0 }
  unit.convert_to(:fur2) { |value| value / 435_600.0 }
  unit.convert_to(:ch2) { |value| value / 4_356.0 }