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    Re: Role of CN at sea, was Re: Averaging sights ... - Chart Errors
    From: Lisa Fiene
    Date: 2004 Oct 18, 11:04 +1000

    Not wanting to harp back to why I believe CN & observation are the
    primary navigational skills a good seaman should possess, with GPS only
    as a backup,
    
    but....(!)
    
    In an article last year in Australia's 'Cruising Helmsman', the
    following was contributed (on the writer's recent passage to the
    Louisiades, which is on the south eastern tip of Papua New Guinea):
    
    "we sailed to a less frequently visited island (Rossel Island) at the
    eastern end of the Louisiade archipelago, and had been concentrating on
    visual navigation as we approaxhed the island.  This was standard
    practice because of hazardous bommies in the area, and we took only a
    passing glance at the navigation system presentation.  ...Two days
    later, we moved to another bay nearby and with gentle sailing conditions
    I noticed a major difference between the navigation system presentation
    and what we were seeing outside 'the office', e.g. the relative location
    of islands, headlands, bearings, etc.  I initially suspected a GPS
    error, and quickly started our backup GPS, but it confirmed our lat/long
    position.  ....By triangulation, I estimated the whole island, reefs
    etc. were displaced by about .7nm to the east.  I then examined our
    previous chart tracks and any fixed points and found that there was an
    increasing chart error as one moved east on AUS382.  It was acceptable
    in the middle of the chart where the more popular anchorages were
    located.  There is a chart warning that while AUS382 has been 'referred
    to the WGS datum; such positions should be moved 0.09 mins south and
    0.06 mins west to agree with this chart".
    
    "As the 0.7nm shift I observed was quite significant and there had been
    a few yachtie tales about map errors and having some difficulty in
    pinpointing openings in the reef in that area, I wrote to the
    Hydrographic Service.  The significant part of their prompt reply follows:
    
    ....It would seem that the topographic content of the chart is indeed
    out of position by approximately 0.5 to 1 minute of longitude (which
    equates to 0.5 to 1nm and therefore correlates with your observations).
     As this is an old chart, it cannot be rectified until it is replaced by
    a new, metric chart.  There is currently no indication as to when this
    will occur.  In the meantime, this office will be issuing an amendment
    to the satellite-derived-positions note to take this inaccuracy into
    account."
    
    As my husband and I are travelling to the Louisiades next May, we are
    convinced that the skills of being able to observe and navigate, and not
    rely totally on GPS for position fixing, are imperitive for any serious
    ocean cruiser.  Even though the GPS information may be  'correct', there
    are still  paper and electronic charts that are not.
    
    Lisa
    
    
    
    
    
    
    
    Frank Reed wrote:
    
    > Herbert Prinz wrote:
    > "In my admittedly very limited experience of ten thousand off shore
    > miles over the last
    > ten years I have not ONCE been in a situation where GPS didn't work,
    > but cel nav
    > would. In fact, I have not once been in a situation at sea where GPS
    > didn't work.
    > Full stop. But I have REPEATEDLY been in situations where celestial was
    > unavailable for several days in a row and GPS was the only position
    > finding tool
    > available. Conclusion: Celestial is not even a backup!"
    >
    > When asked  the question "What's the best backup for GPS?", probably
    > the best answer is "Another GPS!".
    >
    > There is an ambiguity in the above Q&A that may be worth pointing out.
    > The obvious interpretation is that a sailor should carry a spare GPS
    > receiver onboard in addition to the main GPS system. This spare (and
    > maybe a spare for the spare) should be stowed securely in an emergency
    > kit and checked occasionally just as you would check all other
    > emergency gear. But there's another interpretation. What's the best
    > backup for the GPS system itself? And the answer there, too, is
    > another GPS --the whole system. The Europeans, if I remember
    > correctly, are gearing up to spend a small fortune launching an
    > independent GPS-like system. There are plenty of reasons for this, but
    > the most basic is that ANY system should have a backup. And I should
    > note that there is already a similar Russian system, but it seems to
    > be less relevant for practical applications.
    >
    > I think there's a strong analogy in this current situation to the
    > final days of lunars in the latter half of the nineteenth century.
    > Back in 1850 the question would have been "What's the best backup for
    > a ship's chronometer?" And the answer most navigators had settled on
    > by that date was "Another chronometer!". Some small number of
    > navigators continued to count lunars as their emergency backup, but in
    > truth, were they shooting lunars for the same reasons that we shoot
    > lunars today? For fun and challenge and a connection with the past.
    >
    > I think there's something poetic about GPS in the context of the
    > history of the quest for longitude. In the late eighteenth century,
    > the problem of navigation was seen as a great contest between machine
    > and nature, between chronometers and the Moon. Machines won out, of
    > course. GPS rules the world. But a GPS satellite is little more than a
    > an amazingly accurate chronometer, an atomic clock, contained within a
    > tiny artificial moon, a satellite. It's a bit of an unholy union, and
    > yet lunars and chronometers are united in those orbiting clocks.
    >
    > Frank R
    > [ ] Mystic, Connecticut
    > [X] Chicago, Illinois
    
    
    --
    Kind regards
    Lisa Fiene
    ***************************
    
    CopyCare Pacific Pty Ltd
    Lizard Tunes
    ABN 93 101 046 888
    PO Box 314 Ourimbah NSW 2258
    Australia
    Phone/Fax: (02) 43 627 583
    International: 61-2-43 627 583
    E-mail: lisa{at}copycarepacific.com
    Web: www.copycare.com/content/local/ccpaceng.asp
    
    
    

       
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