Welcome to the NavList Message Boards.

NavList:

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

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    or...
       
    Reply
    Re: Great Circle Sailing Chart
    From: David Pike
    Date: 2016 Jan 14, 18:11 -0800

    GL wrote:

    I'm confused. The chart you posted is too small for me to read. You say that the diagram is for determining the "great circle bearing of the destination from the point of departure" but that is easily determined by measuring the initial great circle course at the nearest meridian printed on the chart to the departure. If it is determining  the rhumb line bearing then it is apparently a graphical way of determining the "conversion angle," table 1 in Bowditch.

    I'm not sure that it's as simple as that.  The great circle chart is a planning chart for finding the en-route lat & longs of the great circle between A & B.  No flat chart can represent a spherical chart perfectly.  You always have to give up at least one property of ‘the perfect chart’ to get the property you want most from the chart.  The great circle chart is a gnomonic projection.  Great circles come out as straight lines, but at the expense of losing conformality.  In other words the meridians and parallels frequently don’t cross at right angles; shapes aren’t always preserved; scale varies; and it’s not equal area. This makes measuring angles with a Douglas protractor and distances against a scale somewhere on the chart rather difficult, which is why it’s normally only used as a planning chart.  The simple navigator can take the latitude where the straight line from A to B crosses each 10 degree meridian on the gnomonic chart and transfer those points to a Mercator chart, which is conformal.  Then all you have to do is use your protractor and dividers on the ‘dogleg' sections of track in the normal way, and hopefully you won’t be far out from the true values.  The ‘brainbox’ navigator can use the clever diagrams to do it all on the gnomonic chart.  The actual curves must  be a way of effectively transferring the track angle at each meridian to what it would be at the tangent of contact, where angles are preserved, but it’s a bit beyond me at 01.49AM I’m afraid.  DaveP   

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

    Name:
    (please, no nicknames or handles)
    Email:
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
    Email:

    Email Settings

    Posting Code:

    Custom Index

    Subject:
    Author:
    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site