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 Course via calculator & HO 208
    From: Bruce J. Pennino
    Date: 2016 Oct 17, 18:54 -0400
    It has taken me far too long to finally understand the math for calculating waypoints on a great circle course. I got it! It all “boils –down” to the navigation triangle and the single equation- law of cosines. As described by John Karl, CN in the GPS Age & John Letcher, Jr., Self Contained CN with H.O. 208, the great circle  distance law of cosines equation becomes
    CosD = sinL1sinL2 + cosL1cosL2cos(LongDiff). D is distance in degrees. Multiply by 60 to get great circle distance in NM. Knowing D and rearranging the equation allows calculation of initial course. These equations easily solved with hand calculator or use of tables. BUT what took me so long is that now the navigator’s triangle has the additional information of one angle which never changes for the initial great circle course and waypoints . Now using increments of D,ie, 5 degrees or 300NM,10 degrees 600 NM, etc along the great circle, the latitudes of the waypoints are calculated and then the long. difference can be quickly calculated. Dutton’s method does the same thing but works backwards and forward from the vertex (max. lat.) of the  course . I finally understood all this from Greg’s calcs. Thank you very much.
     
    Dutton’s has one useful equation the latitude of the vertex: Cosine LatVer = CosL1sin C where C is the intial course.  The change in longitude from the starting point to the vertex is sindt =cosC/sin LV.
     
    Practically it seems to me way points only need to be calculated once because this exact course is never precisely followed. As the trip progressed, the navigator would update/change the course based on progress and by quickly calculating a new D and Z with an updated fix.  This would be done as frequently as necessary based on the velocity of the vessel/plane. Tables like H.O. 208 etc could be used for new D & Z . Progress could be compared to the original route. I imagine GPS and auto pilots do this almost continuously. Thanks for all helpful hints. Best regards.,

    Bruce













     
    Sent: Saturday, October 01, 2016 10:33 PM
    Subject: [NavList] Re: Great Circle Course via calculator & HO 208
     

    Bruce,

    See this Pub 229 great circle solution for waypoints 300 nautical miles apart for your example.  Enter the Pub 229 page for LHA 49 ( initial GC course) and Latitude 35°.  90°- distance in degrees to way point is used for the declination entry. Hc = way point Latitude and Z = difference in longitude from departure to Way point. Hopefully this will help with your Pub 208 solution.

    Greg Rudzinski



    Attached File:



    Attached File:



    Attached File:



    View and reply to this message

       
    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