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    Re: Need Engraving Suggestions for Emergency Navigation by "Dog Tag"
    From: Frank Reed
    Date: 2014 Feb 12, 14:16 -0800

    Nick, you wrote:
    "Or given the limitations, would it perhaps be more important to include reminders of the makeshift formulas I already have (given the diminished capacity for recall under stressful conditions)? "

    Yeah, I think something like that would make sense: use that space for mnemonic hints. As everyone else has already indicated, 24 characters is a mighty small block of data. You suggested the possibility of compression, but I assume that we're limited to a fairly basic set of characters so maybe 6 bits per character for a total of 144 bits of data. That's a tiny box!

    Small blocks of data can still be useful. One of the most popular uses for a small block like that is for a checksum or similar data validation check (which, after all, is a kind of "mnemonic"). For example, the majority of credit card numbers include one digit which is derived from all of the other digits according to a simple algorithm (http://en.wikipedia.org/wiki/Luhn_algorithm). This is a simple way of avoiding about 90% of manual data entry errors. So as you suggest, you could use that little block of 24 characters as a means of encoding some data or some sum of data as a means of verifying the things that you have memorized.

    Suppose for example that you have memorized the declinations of 24 stars to the nearest minute of arc. You might list those in alphabetical order and then record the final digit of each one in this small data block. The data would not stand on its own, but it would be a useful check on your memory, maybe validating that you have not swapped any pairs in your head. Or maybe you could come up with some 24-character list of letters and numbers that would remind you of all of the important steps in clearing a sight. For example, if you have been away from celestial navigation for a long time, you might not remember the various corrections that need to be added to a general sight: IC, dip, refraction, temp/pressure correction, parallax, semi-diameter. But if you saw the letters IDRTXS (I used X since it's unique to the word 'parallax' while 'P' might have been confused with 'pressure') then you could recall the list quickly and accurately --an most importantly, with a degree of confidence. This is just an example. You could also include basic rules of the intercept method, like "Hc>Ho,a" (away). This is the sort of thing that you could re-derive by careful thinking when in doubt, but a little hint like this could add confidence.

    Finally, consider using those 24 characters to store the combination for the lock that lets you get at your backup GPS when you realize that you're in too much trouble to be messing around with the stars! :)

    -FER

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