Welcome to the NavList Message Boards.


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

Compose Your Message

Add Images & Files
    Manual slope plotting
    From: Gary LaPook
    Date: 2010 Nov 27, 00:04 -0800

    There has been some discussions lately about using the technique of
    graphing a number of sextant observations on the same body and comparing
    them with a line having a slope representing the change in altitude of
    the body over time to allow discarding erroneous observations so as to
    keep them from distorting an average of the good observations. If you
    are going to do this then, if you are not taking the sights from the
    beach but from a moving vessel, you must adjust the slope of the line to
    account for the movement of the vessel. Back in February I posted a
    table I computed for adjusting sextant observations for the movement of
    the vessel and this same table can be used for adjusting the slope of
    the line for "manual slope plotting" technique. The table is here:
    For even  more precision, and for speeds in the range of .5 to 9, knots
    you can use the Motion of the Observer (MOO) table found in H.O. 249
    simply by dividing the tabulated speeds, 50 to 900 knots, by 100 and at
    the same time dividing the tabulated values for the correction by the
    same factor of 100. There are two tables, one for a one minute period
    and one for a four minute period. I posted excerpts of H.O. 249
    including the MOO tables here:
    You can review my previous posts on how to use these tables at these links:
    You can also use the "Motion of the Body" (MOB) tables in the excerpts
    of H.O. 249 for actually plotting the sloping line representing the
    change in altitude of the body over time. The two MOB tables show this
    for a one minute period and for a four minute period and are equally
    valid standing on the beach, sailing at 6 knots or flying at 900 knots.
    When you study this table you will see that the slope of the line is
    determined solely for the azimuth of the body and the latitude of the
    observer. You can simply refer to this table and then plot the sloping line.
    Think these tables through and you can figure the sign to use for
    plotting the line, (although this should be obvious, bodies to the east
    are rising and those to the west are descending.)
    Give em a try.

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    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 Settings

    Posting Code:

    Custom Index

    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