Re: [Phys-l] polar grid navigation - last words??

[brian whatcott <betwys1@sbcglobal.net>]

On 2/16/2012 2:36 PM, John Denker wrote:
>   ...On real-world navigation charts, there are
>    strategically-located_compass roses_  ... so all you need to do is
>    parallel-transport your vector from a grid-course rose to a suitable
>    true-course rose.  Note that different true-course roses will be
>    aligned differently, depending on longitude, but this is obvious
>    when you look at the chart and can easily be taken into account.
>    This is a good example of analog computing....

Well, I suppose there COULD be polar stereographic charts with "true 
compass roses"
though I even had trouble writing down that curious concept!
Perhaps John had in mind the circular slide rule such as Jeppeson's CR-3 
which can be used for grid navigation.   The meridians drawn converging 
on the pole provide
suitable orientation for local true references. He is certainly familiar 
with the magnetic compass roses printed on low level charts projected on 
the Lambert's conical orthomorphic basis for non polar latitudes.     
These are exceptionally useful where the variation does not alter by ten 
degrees every ten miles
 (Q: how closely  would such a track approach the pole?)

 Instead of central projections onto a cone cutting two reference 
latitudes, the polar stereographic is a central projection on a plane 
tangent at the pole BTW.

But I was more amused by John's other suggestion: to look up  a blog...

"Also note that not all south polar grid navigation charts choose the
antimeridian as grid north (as in the video).  For example, see
   http://www.andrill.org/iceberg/blogs/luann/all.php
and search for "Jess"."


This seems to advocate rotating a south polar stereographic until the image resembles a duck.    Really?   :-)


Brian W