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    Re: How to choose among position lines?
    From: Doug Royer
    Date: 2003 Jun 11, 10:54 -0700
    Federico,in theory one divides the number of observations one is planning to make into 360* to find the proper Zn spread.So,if 8 bodies are to be observed for a round the Zn spread used to minimize error would be 45* between each one.The more sights in a round the less error for the round in theory.In practice it is entirely differant.I've spent the last 20 years in the U.S.M.M.,the last 5 as 2nd officer,and can tell you that with the other duties required there is not time to plan or take that many sights in a round.The maximum I've ever done was a 4 sight round with the Zn's spaced as close to 90* apart as conditions allowed.We've trained and used the 3 sight round because for all practical purposes that spread nulls out most errors and cuts observation times down so other duties can be completed.If the Zn's of the round are not spaced evenly it is advised that one bisects the LOP's to form an MPP.Most of the time a single sight is taken,reduced,advanced and combined with later sights to form a MPP.From: Federico Rossi [mailto:fedeastro.rossi@LIBERO.IT]
    Sent: Wednesday, June 11, 2003 10:08
    Subject: How to choose among position lines?

    Hallo everybody,
    I want to submit you a doubt I often encounter when I draw position lines to find the position.
    Usually, in the Italian merchant marine they used to plot 4 position lines, aproximately 90° with respect to each other, and this configuration doesn't leave space to doubt when it's time to draw the bisecants in order to reduce the systematic errors.
    The Americans used to be even more practical and adopted 3 lines spaced 120° (obviously this in the ideal condition when you can choose which bodies to shoot).
    In "100 problems in Celestial Navigation", which I consider a very good exercise book and I advise to everybody, instead the author often uses 5, 6, 7 or sometimes even 8 bodies, differently spaced in azimuth. In this cases, are there general criteria to choose the lines to use (since the choice can bring to significantly different positions)?
    Federico Rossi
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