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    Re: Fw: Grounding of cruiser PORT ROYAL
    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
    
    
    
    File:


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