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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

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