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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Fw: Grounding of cruiser PORT ROYAL
From: Frank Reed
Date: 2009 Jul 22, 20:05 -0700
From: Frank Reed
Date: 2009 Jul 22, 20:05 -0700
Lu, you asked about my source for charts. For non-navigational use, I generally prefer the older charts. There's an archive of a couple of centuries worth of old US Coast Survey charts here: http://historicalcharts.noaa.gov/historicals/historical_zoom.asp I used a 1996 chart for the diagram in the last post. Since I had already gotten interested in the changes in the charting of that area over the years, I'm attaching (link below) a comparison of a selection of years for the area immediately south of the long runway at the Honolulu airport. The comparison shows that the essential features of the underwater contours and soundings do not change from 1927 through 1996. Notice the red dot on each small chart which represents my best estimate for the spot where the Port Royal got stuck back in February. The shape of the contours right around there is clearly the same all the way through. The 1927 chart does not show soundings beyond the line of breakers. Starting in the late 1960s, you can see indications of the construction of the new long runway, which was built by dredging from the so-called "borrow pits" just on the northeast edge of these chart samples. From 1985 to 1989 there is a sudden change in the latitude and longitude lines. The intersection between them jumps about 1400 feet to the northwest. This reflects the change from the "Old Hawaiian Datum" to a modern datum, which I believe is WGS84. There's no actual change in measured positions, of course. At the bottom is the same area on the modern chart, and while the soundings are roughly the same, the contours are different. If you check the description of the sources of the data on that 2006 chart, they say that this area was last surveyed some 70 years ago at the latest, so the changes on the most recent chart do not necessarily reflect new data (though some changes have to be new data). Instead, I suspect that the new contours are a result of the digitization algorithms that underlie the new charts. I think it's likely that they took the available sounding data, ran those through standard mapping algorithms to generate smooth surfaces, and then generated new contours from those surfaces. That's probably reasonable since the old contours were drawn by hand by charting "artists" to give a general sense of the shape of the bottom based on the soundings. But the soundings are the primary data here. By the way, speaking of that "line of breakers" on the 1927 chart, that could have been a big, obvious clue to those navigating the Port Royal that they were about to plow their vessel onto those shoals. But as it turns out, they had calm seas working against them that night. According to one of the media reports, the risk to the vessel (then sitting on the reef when they were reporting) was relatively low since, as they said, the breaking waves were unusually small at that time. Sure, that's good for the ship once it was stuck on the shoals, but if the waves had been breaking more violently, they could well have provided that "big, obvious clue" that would have kept the vessel from running aground in the first place. Such is luck. -FER --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---