Hmmm. Surely a nautical mile is the same wherever you are. A nauticle mile is 1 minute of latitude - 1 minute of longitude is different distance depending where you are.
Always measure distance on the side of the chart - have I got that right?!
Ah.... Here we go......
Knot (Nauticle Mile)
A knot is equal to one nauticle mile per hour. A nautical mile differs from the statute mile - nautical mile = 6080 feet while a statute mile = 5280 feet (learned it in imperial, don't have the metric conversion). A cable is equal to 1/10th of a nautical mile - i.e. 608 feet, but is generally rounded off to 600 feet. nautical miles are indicated using the (') symbol - the same symbol used for minutes. Makes sense, because one nautical mile = one minute of latitude! Therefore, 60 minutes of latitude (one degree) = 60' - whether your near the equator, the tropic of Cancer or Alert! But you have to be careful when measuring on a chart - Mercator charts are made by wrapping a piece of paper around the earth in a cylinder shape (as opposed to conic or polyconic which use a cone shape or a series of cones) However, Mercator are the most popular chart projection for navigating because rumb lines or course lines are straight, and they are easy to measure distances, bearings and lat and long off. Like any chart, there is distortion - the distance between the latitude lines change and you must be careful to measure distances off the latitude scale closest to where you are transitting. And make sure you know what projection your chart is - you can't use the latitude scale on a Polyconic chart for measuring distances, you have to use the 'sea mile' scale located at the top and the bottom of the chart! (if you have a polyconic chart look at the latitude lines - if you put a set of parallel rulers along them you'll see they aren't straight! As well, the lines of longitude aren't parallel - they converge towards some point off the chart - the point of the cone).