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Geographic Curiosities

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 could run 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 revolutions.

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 its length.

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 questions.

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