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Measuring the size of the earth is an extreme case, because the answer is
so well known. Indeed, you don't even need a highway map! To get a good
estimate of the circumference of the earth, you can just measure the length
of a meter stick (which is pretty easy :-) then multiply by 40,000. Even
in less extreme cases, known quantities manage to insinuate themselves into
measurements in all sorts of ways.