Why is speed measured in knots (as opposed to kmp/h or mp/h), and distances in nautical miles? Is there a reason for following such a system?
To quote from the Wikipedia article on the subject:
Although the unit knot does not fit within the SI system, its retention for nautical and aviation use is important because standard nautical charts are on the Mercator projection and the scale varies with latitude. On a chart of the North Atlantic, the scale varies by a factor of two from Florida to Greenland. A single graphic scale, of the sort on many maps, would therefore be useless on such a chart. Since the length of a nautical mile, for practical purposes, is equivalent to about a minute of latitude, a distance in nautical miles on a chart can easily be measured by using dividers and the latitude scales on the sides of the chart. Recent British Admiralty charts have a latitude scale down the middle to make this even easier.
- e.g. BA Chart 73, Puerto de Huelva and Approaches, 2002
Knots are a measurement of nautical miles per hour. As long as we are using nautical miles for distance for navigation purposes, it would be silly to use anything other than nautical miles per hour.
One nautical mile is one minute of latitude. Maps can therefore provide a pretty accurate measurement of distance even at the higher latitudes as long as there is a lat/long scale. If you were using anything but nautical miles as your measurement, you would have to add an extra step in to convert.
Finally, if it 'aint broke, don't fix it. The current system certainly isnt consistent from country to country. But if you were to change things, the likelihood of everyone agreeing to using the metric or imperial system? It's much easier to stick with this neutral measurement.