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    Re: Traditional navigation by slide rule
    From: Gary LaPook
    Date: 2016 Jan 21, 12:23 +0000
    Let's see. We are used to seeing the azimuth derived in degrees and tenths of a degree because that is the way the standard computation tables give it, HO 214, HO 229 etc. So if all you are doing is doing computations at home for practice and want to compare your result with the "book value" then tenths would be useful. But who on a small boat or even on a large ship actually plot the azimuth to a tenth of a degree accuracy, I'll bet you only do it to the whole degree. so it makes no difference if the computed azimuth comes out in minutes or in tenths of a degree, they all get plotted the same way. And even then, if your results for azimuth come off your slide rule in minutes it is really simple to make that one conversion in you head to tenths. 

    Doing celnav computations,  all the inputs are in degrees and minutes, the latitude, the declination the LHA 
    and the output of Hc is also in degrees and minutes. You then compare your Hc with the reading off the sextant, also in degrees and minutes. So if using a slide rule in decimal degrees all these inputs and outputs have to be converted,  first from degrees and minutes to decimal degrees and then back to degrees and minutes at the end except for that Azimuth. Where are you more likely to make errors.

    But, as I said before, when all the charts display latitude and longitude in decimal degrees, and when the Nautical Almanac publishes the data in that format and when your sextant uses decimal degrees and a new sort of a nautical mile is defined as 1/100th of a degree on the great circle, (185.2 meters) and, for comparison purposes, HO 249 and HO 229 and, if you like using the old methods, HO 208 and HO 211 are republished in decimal format then it will make sense to have a navigational slide rule marked in decimal degrees.


    From: Greg Rudzinski <NoReply_Rudzinski@fer3.com>
    To: garylapook@pacbell.net
    Sent: Wednesday, January 20, 2016 12:32 PM
    Subject: [NavList] Re: Traditional navigation by slide rule

    Degrees and tenths are best for azimuth calculations. Degrees and minutes are best for Hc calculations. So how about both scales.
    Greg Rudzinski
    From: Bob Goethe
    Date: 2016 Jan 20, 10:51 -0800
    I have gotten sufficient control of using Visual Basic together with CorelDraw that I can generate a mathematically precise C-scale for a slide rule with just a handful of lines of code.  And having found that 3-D printing services such as Shapeways exist, this year could be the year for manufacturing a navigator's slide rule.
    Perhaps 2018 could be the year to 3-D print/mass-produce Bygrave cylindrical rules.
    But once you start getting the technical challenges under control, esthetics and usability start coming to the foreground.  John Brown and Gary LaPook have differing senses of what kind of increments would make the trig scales of a slide rule easiest for a navigator to use:  decimal degrees, or degrees and minutes.
    I have attached a graphic to this message that illustrates what a portion of a slide rule tangent scale looks like in decimal degrees (top image), and degrees/minutes (middle image). 
    I have also thrown in at the bottom a picture of the logarithmic C-Scale that I have VBA code generating...with the code making the decisions not just on horizontal placement, but vertical height of the lines.
    There are some of you who are reading these words who say to yourselves, "If Bob ever *does* learn how to 'mass produce' slide rules for navigators, I think I would like one."  Among this group of people, I would find it helpful to get a consensus as to whether decimal degrees or degrees/minutes would be easier to read.  After all, probably 85% of the total world market for the Mark 1 Navigator's Slide Rule is to be found among members of this group.  If the design doesn't suit us, then it is not much good at all.

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