Re: [Paddlewise] Best pitch for owning a GPS

It's not just GPS units that have errors in the positions of rocks, etc.
Regular charts have them too; in fact it's likely that the navigation charts
on GPS units just transferred the errors over from the published charts when
they digitized them.

When we were using sextants we seldom noticed these errors; or at least I
never did. Even with the original electronic navigation systems (Omega,
Loran-A and -C and Transit Satellite) nothing was really accurate to the
degree that we could demonstrate that a chart had errors unless we were a
hydrographic survey vessel; we just had other things to do. But in the past
20 years, since the advent of GPS, I've found several instances of chart
error. It's probably much more evident now with the deliberate errors turned
off.

Admiralty charts were no less immune to these errors than US charts and some
charts from third-world countries (Mexico is one) were - contrary to popular
legend - much better than both US and UK charts. Which reminds me to advise
everyone that the Mexican tide tables for the Sea of Cortez are (or, at
least, were) much more accurate than those published in the USA for that
region.

So I suspect that until all charts have been re-surveyed using modern
techniques (not likely in my lifetime) we should all be very careful of
assuming that the chart on our GPS or chart table or kayak deck is
absolutely accurate.

As an aside; it's interesting to note that GPS satellites have to take into
account the relativistic effects of time dilation in order to give us
accurate positions. I never thought about this until I watched a program
about Albert Einstein over the weekend. Yet more evidence that Professor
Einstein's "thought experiments" worked very well.


Craig Jungers
Royal City, WA

On 1/1/07, Dave Kruger <kdruger_at_pacifier.com> wrote:
>
> Robert Livingston wrote:
>
> > You are assuming that the chart is accurate and they generally are. But
> > you have to recognize that if you are relying on every rock to be within
> > 100 feet of where the GPS would put it that you might run into an area
> > that the map makers were in error. These are not regions that commercial
> > traffic is very interested in. So you have to stay alert.
>
> I'd like to reinforce this.  One day, while gunkholing its shore, I found
> several near-surface underwater rocks just off Gibraltor Island, in the
> Broken Group, Vancouver Island, which were mis-charted by 100 feet or
> more.
>   Not an issue for me that day, but a good caveat for future dependency on
>
> a GPS.  Better to mainly depend on your eyes, and glance at the GPS now
> and
> then to confirm what you see, or to see if there is a hidden rock in the
> vicinity.
>
> Robert's description of using his GPS to route-find through a reef maze to
>
> a safe harbor was a telling illustration, also, of the value of a unit.
> I'd want a chart on deck, too, in case the thing cratered mid-passage, so
> I
> could patch my way through the old way.  I really like charts, and enjoy
> matching what is on the chart to what I see.  For me, the GPS is mostly a
> tool.  Some days, I leave it home, to get away from looking at that blame
> screen!
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