# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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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.

gl
--------------------------------------------
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

For
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
used.

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

View
```
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