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    Re: Great Circle Sailing Chart
    From: Gary LaPook
    Date: 2016 Jan 15, 18:20 +0000

    Well, I've been wrong about things before and I'm sure I will be wrong about 
    things again, this has been one of those times. Thanks for the correction.
    On Fri, 1/15/16, David Pike  wrote:
     Subject: [NavList] Re: Great Circle Sailing Chart
     To: garylapook@pacbell.net
     Date: Friday, January 15, 2016, 7:22 AM
     practical navigation just place your plotter's straight
     edge on the straight line of the GC on the gnomic chart and
     read out the course against the closest meridian to the
     departure. Then slide the plotter along and read the courses
     at other meridians. Just like reading the course on standard
     flight charts. glYou can’t always do
     that on a gnomonic chart Gary.  The example doesn’t look too bad, because the eye
     is taken to the centre of the chart, which is also the
     tangent of contact, but the lack of orthagonality gets worse
     the closer you get to the edges of the chart. 
     Look in the SW
     corner.  Where would you sit your Douglas protractor?  In the sectors 360-090 and 180-270,
     ninety degrees indicated on the chart is clearly less than
     ninety degrees on a protractor, and in the sectors 090-180
     and 270-360 is more than ninety degrees measured with a
     protractor.  I.e. the chart isn’t conformal, and the normal
     techniques of chartwork can’t always be
     Just to add to my earlier
     comments, which were written in haste well after my normal
     bedtime.  Gnomonic charts aren’t just used to plan
     navigation trips, although Lindbergh used one to
     plan his Atlantic flight very
     effectively.  They can be used for any activity which involves
     great circles.  E.g. pointing directional aerials or pinpointing
     seismic activity.  For navigational use, the lack of orthogonality
     means that angles aren’t preserved, shape isn’t
     preserved, and scale alters at different rates in different
     directions.  By way of a contrast, the Mercator chart is
     conformal.  Although scale changes, it does so at the same rate
     in each direction so shape is preserved at the expense of
     the chart not being equal area.  E,g.  On some Mercators,
     Greenland appears nearly as big as Africa.  DaveP
     and reply to this message

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