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While working the photo analysis puzzle for the USS Port Royal grounding, I 
noticed that there is a sudden change in the latitude and longitude grid on 
nautical charts for those reefs south of the airport. It occurs in the 1980s. 
It turns out that this is a shift from the "Old Hawaiian Datum" so-called to 
a modern mapping datum. It amounts to a shift in the latitude lines of about 
10 arcseconds and the longitude lines of about 9 arcseconds (there are exact 
numbers on the chart with a precision of a thousandth of an arcsecond which 
may be "just a bit" beyond any practical application --that's about one inch, 
by the way).

So here's my question. If you shoot the altitude of the Sun at noon on the 
south shore of Oahu and do all the usual corrections with exacting care, 
which latitude would match the chart? Those ten arcseconds of difference 
between the two datums are distinguishable with good sextant observations, 
especially if using an artificial horizon and averaged over a decent number 
of sights. Would the "Old Hawaiian Datum" match the stars?

Also, it's interesting to note that modern charts of the reefs, shoals, and 
depths generally in that area where the Port Royal grounded are EXACTLY the 
same on the nautical chart from 1996 as on the rather primitive chart from 
1927. It's rather amazing that there were no better surveys in the 
intervening seven decades. I don't know if there's a better chart after 1996.

-FER




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