So important, indeed, is the use of maps in geographic work, that, without wishing to propose any new law, it seems fair to suggest to the geographer a ready rule of thumb to test the geographic quality of any study he is making: if his problem cannot be studied fundamentally by maps - usually by a comparison of several maps - then it is questionable whether or not it is within the field of geography.
p. 425 - The Nature of Geography (1939)