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post #6 of Old 10-04-2012
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Re: Great Lakes Boaters, Polyconic charts, mph

Originally Posted by Hartley18 View Post
Came across this old thread whilst poking around. Looks like an interesting part of the world to sail in - you're lucky!

Polyconic projection is an interesting subject and certainly not something I've come across before, but I thought I'd correct an error in the statements below in case anyone else happened by and came to grief as a result:

The charts listed above (and presumably most others you own) are all clearly marked in Nautical Miles along both edges in the standard fashion of nautical charts of every other corner of the globe. NOAA have helpfully added Statute mile scales and a host of other relevant info for you to use if you want to.

Over such a small area of the planet you're free to use miles for your navigation if you want to - but to categorically state that the charts used are in Statute miles and "Nautical mile just makes no sense" simply is not correct.

The OP is correct; on a polyconic or conic chart distances are measured with a distance scale, not the latitude scale. The distance scale on both charts is statute and some of the cautionary notes notes indicate m.p.h., not knots.

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