Q: How long is the Mississippi river?
A: This is a tough question. Many years ago a
famous mathematician wrote a big essay on this: “How long is the coast
of Britain?” And scientists still don't have an answer.
Natural shapes like coastlines and
rivers don't have a definite length. For one thing they're constantly
changing. But even if they were stable, it would be impossible to
measure them with certainty.
How do you measure a curve on a map anyway? You
a string along the curve, then stretch the string over a ruler and
measure it. Or you could use a map measurer, a device with a tiny wheel
that you run over the curve, and a dial that shows the number of
Let's assume you could run that string or map
measurer exactly over the curve. Then we'd have to step back
and ask the more important question: is the map totally accurate?
Where exactly does the Mississippi begin? Where exactly does it
end? Does the blue line on the map show every little bend in the river,
around every rock on its bed? Of course not. But all those bends add to
If you measured the Mississippi from a folding
AAA-type travel map of the U.S., you'd get one answer. If you
measured it off a large wall map of the U.S., it could be twice
as long. If you picked out the topographic sheets and measured those,
it might be ten times longer. And so on. So which number do we believe?
Maps actually have a lot of little errors,
generalizations and uncertainties in
them. Even computer maps do. No map could show the exact course of the
Mississippi. Or the coastline of Britain. You can state
these measurements only with a number of assumptions, e.g. “as of 2006
January 1,” “measured at a scale of 1:50,000,” “at high tide,” etc. You
can compare the lengths of two rivers or coastlines if the
measurement scales and methods are the same—or if the lengths are so
different that the methods and measurements don't matter anyway.
This is one of the joys of being a geographer.
There's a lot of data out there, but getting at the truth can be a
challenge. You have to plant your feet in reality and ask tough