Hi Wouter,

I use SI units for most engineering calculations at work and then convert back to INCH|POUND when required. There are fewer constants and conversions to remember so I am less likely to screw it all up. BUT - for navigation I use nautical miles.

When you are trying to figure the distance between 2 points on the surface of the earth you start with the lattitude and longitude and then use "the sailings". Which type you use will depend on the distance, direcion and desired accuracy of the calculation. Unless you are only concerned with a very short distance you would not use a plain sailing. You would typically use a mercator sailing to find the course and distance. The table (table 6 in Bowditch) to get the meridional parts is all based on degrees and minutes. This makes using nautical miles the most convenient.

Now, you could get really clever and do your mercator sailings purely by calculation, based on some trig and the WGS ellipsiod. But, how do you calculate distance on the surface of an ellipsiod and not use polar coordinates at some point? If you are going to use degrees and minutes to measure angle then nautical miles require the least conversion.

Aircraft navigation uses nautical miles for the same reasons. The speed over ground is used for navigation.

If I am measuring distance in nautical miles, it makes sense to measure speed in the same units.

Talk to you later
-colin

Steve- I have not logged in for a long time. I just saw your message from December.