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    Re: NOW what?
    From: Bill B
    Date: 2015 Aug 15, 16:43 -0400

    On 8/14/2015 9:11 PM, Brad Morris wrote:
    > Hello Bill
    >
    > I'm not sure what to make of your assessment of Dutton's.  Did you
    > approve of Dutton or think it not right?
    > Brad
    
    Would seldom second guess you, or supplement your recommendation with
    one of my own if I thought them not right.
    
    Short answer long (Subtitled: I didn't see that coming!)
    
    I found Dutton's most enlightening, covering topics I had never
    considered as a small-craft navigator. For example, calculations on how
    far ahead of the actual turning point in a tight channel etc. a large
    ship would have to initiate it's turn in order to actually execute the
    maneuver at the desired spot.
    
    The same for "How to Read a Nautical Chart." A seemingly benign topic
    covered in most part by NOAA's Chart No. 1. Calder goes into depth on
    many topics, including the (extreme) age of some charts, levels of
    accuracy, classes of hydrographic surveys and minimum standards, methods
    of scanning the bottom which may have missed huge submerged objects,
    local datum in certain areas, and some of the shortcomings of electronic
    chart-plotter data and layers. (If you followed the recent VOR, think
    Team Vesta Wind finding a needle in a haystack and augering in, or a
    sail-training ship grounding on an "uncharted sand bar" a stones throw
    from shore.)
    
    A little nugget on GPS elevation caught my attention. Often when we
    speak of elevations reported by our civilian GPS units as being off by
    10 or 100 meters, it is dismissed by claiming 50 meters horizontally is
    no big deal, but a 50 meter drop could be.
    
    GPS models the Earth as an ellipsoid--the sea-level surface of the
    world--and uses that as a reference for height. Calder explains the
    geoid surface does not necessarily coincide with the ellipsoidal or
    topographical surface. ("However, because the geoid undulates in a
    mathematically unpredictable fashion, there is no mathematical
    relationship between the ellipsoid and the geoid....Global differences
    between the geoid and the WGS 84 ellipsoid range from as much as +78
    meters [+257 feet] in the region of Papua New Guinea to  -103 meters
    [-340 feet] in the Indiana Ocean off India and Sri Lanka.") I feel
    confident Frank can expound on the above now that he's covered gravity
    wells and (if memory  serves) island gravity mounds :-)
    
    (If you want to add memory to and hack your civilian GPS, Calder notes
    that in the United States geoid models can be downloaded from
    www.ngs.noaa.gov/geoid ;-)
    
    The long and the short of it, I would highly recommend both books as
    fascinating winter reads, or as a necessity for those piloting outside
    their area of local knowledge.
    
    
    
    
    

       
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