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    Re: Converting degrees-minutes to degrees and back
    From: Brad Morris
    Date: 2018 Nov 20, 08:54 -0500

    The form of the number is only a convenience.  

    Do you want it in Degrees, Minutes, Seconds and Thirds (I have indeed seen references to thirds in some very old books).

    Do you want it in Deg, Minutes and seconds?  Deg, Minutes.tenths?

    My problem with deg, min.tenths is that my top notch vernier metal sextant is resolved to 10", which is not a tenth.  My Circle of Reflection has two verniers, which are to be averaged to minimize the centering error.  As each is resolved to 10", when averaged you resolve to 5".  That's not a tenth either.  Am I to abandon these excellent, world class instruments because some "person" doesn't view the arithmetic as to his tastes? 

    Because some sextants use micrometer drums resolved to tenths?

    Who, afterall, is the arbiter of good taste?  I can state unequivocally that it is Petronious Arbiter, who decided such matters in Nero's court (yes, that one).  Too bad he didn't rule on that equally important feet/meters issue or on the form of the angular reading.  Then we would know which one is "right".


    On Mon, Nov 19, 2018, 10:02 AM Brad Morris <NoReply_Morris@fer3.com wrote:

    There is no "rule" in today's world.  Most computations are done digitally (electronically).  For that reason alone, computations are done in pure decimal degrees.  The conversions back and forth are done simply for the convenience of the user.

    The one tricky bit is the conversion with negative values, particularly with spreadsheets.  With a little bit of care, however, there is no concern.  Just assure yourself that the conversion is functional.  

    On the otherhand, if you are converting back and forth manually, then in my view, this is very error prone.  More so, in my opinion, than sexagesimal arithmetic of addition and subtraction.  The conversions use multiplication and division.   Perhaps you can do these conversions in your head, but I cannot.


    On Mon, Nov 19, 2018, 12:55 AM Steve E. Bryant <NoReply_Bryant@fer3.com wrote:

    When performing calculations involving degrees and minutes, I much prefer converting the minutes to degrees, proceeding with the additions and subtractions, and then converting the decimal degrees back to minutes.  

    There may be some statistical reason for not doing it that way; or, is the practice perfectly acceptable.   

    There are those professionals who teach navigation that insist upon working only with degrees and minutes, unless it's necessary to use decimal degrees for calculator entry in which case they carry the decimal out to the fifth place.  

    What are the rules, if any, that make the degree/minute conversions unacceptable; and, if there are prohibitions against the practice, is there anything one can do in the process that will statistically justify the conversions?   

    Steve OCBC


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