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    Re: Magnetic Variation (Declination) Calculator
    From: Lu Abel
    Date: 2009 Aug 04, 15:33 -0700
    Greg, thanks for the comment.   I was going to point out to Brad that there are two commonplace problems experienced with formulas, at least one of which probably lies at the heart of the problem with the declination web site:

    1.  Some fairly common formulas, especially trigonometric ones, often have "singularity points" where the answer blows up or becomes undefined.  I suspect this is the case with the declination web site; the calculations have likely been programmed to return "undefined" as an answer for latitudes sufficiently close to 90 degrees which is then displayed as zero.  

    As another example, try calculating the initial course for a great circle between two antipodal points (ones opposite each other on the globe) -- any course is a great circle!

    2.  Even when the mathematical formula doesn't blow up, they always are for calculations of infinite precision.  Computers and calculators don't have an infinite number of digits and strange things can happen as a result.  For example, the most common formula for great-circle distance requires the subtraction of two numbers.   These numbers become nearly equal as the two points approach each other.  Most calculators and computers produce gibberish answers when asked, for example, for the great circle distance between two points 100 feet apart.  That's why all GPS units use an alternate formula involving haversines to compute great circle distance; the numbers stay tractable down to zero distance.

    Lu Abel

    Greg Rudzinski wrote:
    Brad,
    
    Manoj C. Nair of NOAA University of Colorado responded to my email
    reguarding the zero result for 90° of latitude and said that the
    current program recognizes latitudes above 89.999 as undefined. The
    zero result must be a default number. I noticed another program glitch
    where 13° West declination was displayed as 167° East.
    
    Greg
    
    On Aug 4, 10:20 am, Brad Morris <bmor...{at}tactronics.com> wrote:
      
    Hi Greg
    
    I just entered the pole for a gee-wiz kind of exercise.  To my surprise, it returned zero, so I tried a location close to the pole.  That returned non-zero, indicating that the software has some sort of bug.
    
    Since I have no way to verify any other result it may present to me, I will distrust the data.  How many other lat long locations give 'bad' data?  How would I know?
    
    Should NOAA fix this?  Certainly. How silly of them to permit an entry which provides erroneous results.
    
    Do navigators depend exclusively on a compass within 366 feet of the north pole?  I sure hope not...
    
    Best Regards
    Brad
    
    -----Original Message-----
    From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Greg Rudzinski
    Sent: Tuesday, August 04, 2009 1:03 PM
    To: NavList
    Subject: [NavList 9329] Re: Magnetic Variation (Declination) Calculator
    
    Brad,
    
    It looks like values above 89.998996° of latitude are all zero instead
    of 14° 24'W. Does NOAA need to correct this since it is only 366 ft
    from the pole?
    
    Greg
    
    "Confidentiality and Privilege Notice
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