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    Re: Marq St. Hilaire - Altitude intercept method
    From: John Cole
    Date: 2007 Oct 26, 17:28 -0500

    Andres is exactly right when he says "write(ing) without rigor can confuse
    to the beginners in CN."  Another list member (I think it was Frank)
    recently pointed out that correct use of terminology is important in
    communicating with technically trained people. Yes indeed. George frequently
    takes to task misquotes and sloppy use of terminology, and rightly so.
    Having said that, most everyone knows the difference between CN on land in
    an armchair and on a pitching boat in a gale and of course it is true that
    in the practical world approximations and short cuts are made that are
    entirely appropriate for those skilled in the art. (I wonder about the $12
    watch though). But in presenting the subject to an audience I think the
    presenter should first state the rigorous case using correct terminology and
    then show how and why it may be simplified in practice or under special
    circumstances.
    
    I find the back and forth banter between members highly educational as it
    nearly always teases out the subject in a way that would never happen in
    most classrooms, and I appreciate the huge amount of time and effort that
    they devote to the task. I am curious as to kind of jobs they have that
    provide the time to research and write this excellent stuff.
    
    John Cole
      
    > From: Andres Ruiz 
    > Reply-To: 
    > Date: Fri, 26 Oct 2007 10:46:55 +0200
    > To: 
    > Conversation: Marq St. Hilaire - Altitude intercept method
    > Subject: [NavList 3621] Re: Marq St. Hilaire - Altitude intercept method
    > 
    > 
    > Gary, my question for you is: are you saying I and writing silly things????
    > No mere controversy please. You can retract your question!
    > 
    > Language is for providing communication to people. Not is the same:
    > "St. Hilaire's method created the exact same LOP as Sumner's method" or
    > "Sumner and St. Hilaire methods produce the same LOP" or -In the practice of
    > old celestial navigation the result of using Sumner or St-Hilaire LoPs is the
    > near the same assumed usual circumstances.-
    > 
    > 2+2= 4
    > 1.99+2.001 approx 4
    > 
    > Anybody with good background knowledge in navigation can filter the
    > information and read between the lines, but write without rigor can confuse to
    > the beginners in CN.
    > 
    > Of course navigation is an art, but also a science and the advances in
    > sciences arise by understanding previous knowledge and acquiring new one.
    > Are you agree?
    > 
    > To test the discrepancy between the to lines, imagine you are in the middle of
    > the North Atlantic Ocean and a two days storm apart from your course several
    > miles away. Now your DR position is far away from the true one. The
    > differences of the results using the two methods depends on the assumed
    > position, and increase, as the distance between the true and assumed position
    > do.
    > 
    > ________________________________________
    > De: NavList@fer3.com [mailto:NavList@fer3.com] En nombre de
    > Gary J. LaPook
    > Enviado el: viernes, 26 de octubre de 2007 9:48
    > Para: NavList@fer3.com
    > Asunto: [NavList 3620] Re: Marq St. Hilaire - Altitude intercept method
    > 
    > Gary LaPook writes:
    > 
    > My question for you is "how many angels� to you want to get dancing on the
    > head of your pin?"
    > 
    > You are right, one line is a chord and one is a tangent but to the available
    > level of accuracy of measuring the altitude and of plotting it on a chart they
    > are the same for all practical purposes. For example, if you plotted two
    > positions determined by the Sumner method 30 NM apart and drew the line
    > between them making one LOP. Then you calculate the same LOP using St. Hilaire
    > for a spot in the center of the two Sumner positions you are right, you would
    > end up with two different LOPs that parallel each other. However, for any
    > altitude up to 77� they would be less than .1 NM apart, that's right, one
    > tenth of one nautical mile! How thick is the line your pencil makes on the
    > chart? How accurate are all of your sextant sights? Do you always achieve one
    > tenth of a minute accuracy? People on this list talk about getting .5 minute
    > of accuracy as a very good result on a boat and with that level of accuracy
    > the difference between a Sumner line and a St. Hilaire line only becomes an
    > issue for sights above 86�. How many times have you taken sights above 86�?
    > Navigation is the art and science of directing a vessel or aircraft safely
    > from one place to another, it is not a mathematical exercise that you do at
    > home on a computer to an accuracy of 42 significant figures. And I don't use
    > "the analytic equation of each LoP on a Mercator chart" I draw a pencil line
    > that has thickness and some level of inaccuracy in azimuth. Everything plotted
    > on a chart is an approximation to some extent and for navigation purposes (not
    > theoretical discussion purposes) the Sumner and St. Hilaire methods produce
    > the same LOP.
    > 
    > gl
    > 
    > 
    > Andres Ruiz wrote:
    > In messages: [NavList 3572], [NavList 3588], [NavList 3596] There is an error
    > of concept.
    > 
    > Gary is not absolutely correct.
    >   
    > St. Hilaire's method created the exact same LOP as Sumner's method but
    > only required doing the computation one time, saving work and reducing
    > the chance for an error.
    >     
    > 
    > The Sumner and the St-Hilaire LoPs are not the same. One is secant to the
    > circle of equal altitude and other is tangent. This means that the points in
    > common between the CoP and the each LoP are different. If You get the analytic
    > equation of each LoP on a Mercator chart You can see that the two lines are
    > not the same.
    > 
    > For more details see the attached PDF.
    > 
    > Please, I want to hear more opinions.
    > 
    > Andr�s Ruiz
    > Navigational Algorithms
    > http://www.geocities.com/andresruizgonzalez
    > 
    > 
    >   
    > 
    > 
    > 
    > > 
    
    
    
    
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