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    Re: Role of CN at sea, was Re: Averaging sights ... - Chart Errors
    From: Trevor Kenchington
    Date: 2004 Oct 17, 22:34 +0000

    Lisa Fiene wrote:
    > 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. [snip]
    It has often been pointed out that, in GPS navigation, you can know your
    own position with extreme precision but that the locations of the
    hazards around you are often much less well known. That is a reversal of
    the situation with non-electronic navigation, in which your estimates of
    your own position are generally much less precise than the chart.
    I don't see that as reducing the value of GPS relative to the
    non-electronic navigational tools but it does illustrate one reason why
    GPS navigation needs more skill than simply turning the system on and
    following the calculated bearing to the next waypoint.
    It is also a warning to navigators properly trained in non-electronic
    position-fixing that GPS is not simply an alternative way of finding
    your position. Some old certainties change in unexpected ways as
    technology advances.
    Trevor Kenchington
    Trevor J. Kenchington PhD                         Gadus@iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
                         Science Serving the Fisheries

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