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: Rejecting outliers
    From: Gary LaPook
    Date: 2011 Jan 8, 10:54 -0800

    But for calculating the slope the DR doesn't need to be particularly accurate as the slope is determined only by your latitude and the azimuth of the body, which you can measure to sufficient accuracy. If you scan the Motion Of the Body table from H.O. 249 at:

    http://www.fer3.com/arc/img/102321.mob%201.pdf

    you will see that the rate of change in altitude is very insensitve to errors in latitude and azimuth. For example, looking at the MOB table for my data where my DR latitude was 14° 25' and the azimuth was 103° true, the tabulated values for the change in altitude for one minute of time changed only 0.1' for a 2 degree azimuth change and by only 0.2' for a 5 degree change in latitude. So  for a five minute observation period, if my DR latitude had been wrong by five degrees the slope would be off by only one minute of altitude. Similarily, the measured azimuth would have had to have been off by four degrees to produce a one minute error in the slope. Scanning H.O. 249, volume 2 shows that to produce an error of four degrees in the azimuth my DR longitude would have had to have been off by ten degrees, 600 nm!

    gl

    --- On Sat, 1/8/11, P H <pmh099---.com> wrote:

    From: P H <pmh099---.com>
    Subject: [NavList] Re: Rejecting outliers
    To: NavList@fer3.com
    Date: Saturday, January 8, 2011, 8:41 AM

    George,

    If you trust your DR then by all means use a precomputed slope.  My spreadsheet is just another tool that the navigator may use at his or her discretion.  We already agreed that one should apply the methods best suited for the given circumstances and use all available and reliable information.


    Peter Hakel


    From: George Huxtable <george{at}hux.me.uk>
    To: NavList@fer3.com
    Sent: Sat, January 8, 2011 8:14:08 AM
    Subject: [NavList] Re: Rejecting outliers

    With regard to Peter Fogg's sequence of nine altitudes, Peter Hakel's
    computer analysis deduced, and then employed, a slope value of about 24'
    (in 5 minutes) whereas the evidence surrounding the taking of the
    observation implied that the slope couldn't in fact differ much from 32'.
    So I asked-

    Is Peter prepared to defend his value [of 24] against the other [of 32]?

    To which Peter Hakel has replied-

    You provided the best defense when you wrote:

    "So let's have a look at the data that Peter Hakel's estimate was based on,
    in the attachment. I would agree that the better fit, with no other
    information to go on than those plotted point, would be the continuous line
    at a slope of 24. Peter's program says so, an Excel fit says so, and my eye
    says so."

    But he seems to have missed the point I was trying to make, and perhaps I
    didn't make it clearly.

    What I was trying to say was that, in assessing the sequence of altitudes,
    all three, Peter's program, an Excel straight line best fit, and my own eye
    would agree that the true rate of change of altitude would have been 24'
    over 5 minutes. AND WE WOULD ALL THREE BE WRONG! Because, from the other
    information we had been given, that slope simply had to be 32. And indeed,
    a slope of 32 would be perfectly compatible with the data, though not the
    best fit to it. There was so much scatter in the altitudes, over that short
    observation time, that a wide range of slopes could fit it. Indeed, those
    observations provided a rotten basis on which to determine the slope. Which
    is why that analysis gives such a wrong answer for the slope.

    George.

    contact George Huxtable, at george{at}hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.





       
    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