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    Re: US. Navy fix accuracy and Interval.
    From: Lu Abel
    Date: 2013 Mar 26, 09:53 -0700
    What fascinates me is how submarine navigation is intrinsically based on the assumption that if you're in deep-water ocean there are no underwater obstacles.   In the early days of nuclear submarines there were several collisions between subs and uncharted underwater seamounts.   I wonder if after the first one or two if only the lowest of the enlisted sailors got to sleep in the berths in the bow of the sub...

    On 3/26/2013 8:29 AM, Frank Reed wrote:

    It's amazing when you stop and consider that the surface of the planet Mars has been mapped in vastly greater detail than 70% of the Earth's surface. If you want to map the ocean floor, you have to sail over it and get an echo off the bottom. Modern technology has barely changed this. There are numerous digital products (like Google Earth, for example) that display hydrographic data and underwater "topography" and create the illusion that this data is accurate and complete on a much finer scale than it really is. But much of this "data" has been derived from indirect sources and represents little more than some algorithm's "best guess". Naturally in shallow waters, where it matters more, the charts are usually based on real grid surveys, but they could be decades old, and you never know whether they were competently prepared. I presume the electronic chart systems usually indicate the age of the hydrographic survey data. Do they prominently indicate the source, too? Or do you have to dig down into the menus for that information?


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