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    Re: Polhemus computer
    From: Frank Reed
    Date: 2017 Aug 6, 13:35 -0700

    John Blake, you wrote:

    One volume of HO229 is for latitudes 0-45 degrees and the other is for latitudes 46-90. If we were on a training mission where we knew that we would not be going north of 45, we only carried one.

    John, this is a trivial matter, of course, but it's one of those bits of trivia that's central to the story, so pardon me for harping on this. The tables you're describing were part of HO249. They were volumes 2/3. That's not the same as HO229.

    As I noted in my earlier post, those volumes 2 and 3 of pub.249 were so very different from volume 1 of 249 that it's easy to think that they must have been an entirely different publication, and indeed it would have made sense if the H.O. (the "Hydrographic Office" which was responsible for numbering these things back then) had given a different number to volumes 2/3. But they did not. They were listed as part of HO249. The relatively similar tables known as HO229 were much denser and over-precise to a level beyond any real practical requirement. They were intended for marine navigation, not air navigation, though a hopelessly meticulous air navigator could have used 229 in the 1970s. Those 229 tables consisted of six volumes divided by latitude (and actually each "volume" was two complete, separate books in the same binding). These were published beginning in the early 1970s. You, John, absolutely did not use 229 if you were navigating in the early 1960s. What you have described are volumes 2 and 3 of HO249. By the way, you're not alone in getting these numbers confused, and it's really a shame that these are the dull and undescriptive names we are stuck with for these products. But we are stuck! 

    Frank Reed
    Clockwork Mapping / ReedNavigation.com
    Conanicut Island USA

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