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    Re: Portuguese shipwreck question
    From: George Huxtable
    Date: 2009 Nov 2, 09:08 -0000

    Thanks to John Huth for providing details of the paper his information on 
    ancient lats and longs came from, as follows-
    
    "It's actually a journal article written in Isis.   Isis does have an online
    archive, so you can dig it up free of charge, but I believe you have to
    register to get it:
    
     John K. Wright, �Notes on the knowledge of latitudes and longitudes in the
    middle ages,�  Isis, Vol. 5, No. 1 (1923) pp. 75-98"
    
    I've contacted the website of the journal, Isis, at 
    http://www.journals.uchicago.edu/loi/isis, which lists a lot of 
    back-numbers, but volume 5 is missing from that list. It also seems, as far 
    as I can tell, that where such back numbers are available, on-line access 
    isn't free, except to members of certain subscribing institutions. Not to 
    the ordinary-Joe, such as I am.
    
    The full run of Isis is available at the Bodleian Library in Oxford, so next 
    visit, I'll take a look, and maybe a copy.
    
    ================
    
    Mercator mapping didn't exist at the time of the Toledan tables, but if John 
    wishes to display the positions on a Mercator map background (which seems 
    sensible), he should first convert the latitudes to a Mercator scale of 
    meridional parts, and plot those numbers against degrees of longitude. This 
    can be done, to quite sufficient accuracy (it ignores the Earth's 
    ellipticity) by applying an Excel function such as-
    =131.9*LOG10(TAN(RADIANS(45+lat/2)))
    
    This would provide an x-y plot on top of which to fit a suitably-scaled 
    Mercator map of that part of the mediaeval world.
    
    On the other hand, if John considers that modern mapping provides a suitable 
    background on which to overlay these locations, he could plot the points 
    onto a map such as that available from OMC (online map creation), at-
    http://www.aquarius.ifm-geomar.de/
    This will provide a modern map of any area he wishes, with a choice of 
    projections (though Mercator may well be most appropriate). It will provide 
    coastal outlines and other features, with a grid, as requested, in a 
    vector-graphics format. On that it will plot his labelled points, defined by 
    longitude and latitude, which can be pasted-in from a spreadsheet. 
    Longitudes would, of course, have to be adjusted from their zero in the 
    Canaries to be based on Greenwich instead.
    
    That program has some awkwardnesses, but I have some experience in getting 
    round these, and may be able to help.
    
    George.
    
    contact George Huxtable, at  george{at}hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ----- Original Message ----- 
    From: "Apache Runner" 
    To: 
    Sent: Sunday, November 01, 2009 10:18 PM
    Subject: [NavList 10375] Re: Portuguese shipwreck question
    
    
    It's actually a journal article written in Isis.   Isis does have an online
    archive, so you can dig it up free of charge, but I believe you have to
    register to get it:
    
     John K. Wright, �Notes on the knowledge of latitudes and longitudes in the
    middle ages,�  Isis, Vol. 5, No. 1 (1923) pp. 75-98
    
    I just copied his table out of that article.   The scatter plots were my
    first crude attempts to see if I could figure out what these places
    corresponded to.   I didn't get as far as actually putting onto a Mercator
    projection, however.   If you know of some software were one can input these
    points onto a Mercator projection, that would be nice, although I can
    probably cobble something together using Mathematica.
    
    Attached is an attempt to draw a map on a rectilinear grid (not a Mercator)
    and my guesses as to some of the place names - modernized.
    
    
    
    
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