(that is another measurement i dont understand)

You may be unfamiliar with it, but you

**can **understand it! (because it is so simple)

1 degree = 60 minutes, .0.5° = 30 minutes, 0.25° = 15 minutes

1 minute = 60 seconds etc etc.

It's primary use is for quick, in-the-head*, maths for navigation. Powers of 60 factor (i.e. can be divided exactly) by 2,3,4,5,6 etc. making division by those factors easy and accurate.

eg. 25° 34' 18" divided by three = 8° 31' 29"

The same units are used on charts and navigating instruments so there's no need to convert.

In general, converting units should be avoided (or left to a computer ;)) as can easily lead to rounding or typographical errors, so if the standard says "inches per foot" stick with it

Bill

*fractions (e.g. of an inch) have some of the same 'magic' which is why they been around so long - It's only the wide spread use of calculating machines that makes metric/decimal systems attractive.