NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Old Hawaiian Datum
From: Frank Reed
Date: 2009 Jul 21, 05:13 -0700
From: Frank Reed
Date: 2009 Jul 21, 05:13 -0700
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 --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---