RE: [Paddlewise] GPS

Jim wrote: -
>Am I becoming a dinosaur by holding on to my
>paper chart and deck compass and never planning
>on giving them up?  A GPS is a very useful tool.

G'Day Jim and Paddlewise,

Here's an account I wrote up some months ago of a navigation training
session during which we compared dead reckoning with GPS: -

Was it 1.8km to the Red Point campsite turn off? or was it 2.7km? The
directions had been carefully measured by GPS. Latitude and longitude were
supplied. And yet that puzzling discrepancy! Anyway our coterie of
navigation students passed the first test. It was a good trip - everyone
arrived!

So Saturday morning and we're all parked around two tables peering at maps
and puzzling over protractors, while Stuart gives an introduction to
navigation with a thought provoking description of dead reckoning and how it
was used on long trips when you just had to get it right; plus a hair
raising tale of what can go wrong when you rely too much on a GPS. It
involved a naked blue man, standing on the beach, trying to attract the
attention of an aged and perturbed couple. You had to be their to
understand - but I'm sure if you ask nicely Stuart will repeat the story.

And then we went to our kayaks and Stuart had us line all 14 of them up to
magnetic north east. Well either Red Point shows alarming geomagnetic
anomalies or we had learnt a lesson in how variable compasses can be. The
lesson was to be careful with those cans of baked beans, those radios and
metallic sail fittings and check the deck compass against a quality hand
bearing compass.

Finally we puzzled away on paper, trying to locate the Shangri La's of
Jervis Bay, the elusive POINT A and the mystical POINT B. That well known
navigational witch, Dee, applied her Portland Square dexterously and
finished hours ahead of the rest of us, while we fussed with set-squares and
protractors and struggled over the precise meaning of "east is least and
west is best".

And so it was we found ourselves paddling many kilometers along a variety of
courses, which we have all sworn to keep secret. Then after much skillful
navigation and accompanied by twenty dolphins playing in our wake, we
arrived at a point within Jervis Bay of unmatched beauty and serenity, a
point in which golden shafts of sunlight played and refracted and where
Mariners, Mirages, Nadgees and Pittaraks were at peace together.

A place, which we were told was TOTALLY the wrong place. Out by 900 meters
if you checked by GPS.

Was this the self same GPS which measured the turn off at 1.8km instead of
2.7km??

We analysed our results and the average dead reckoning error over a 10km
paddle was 900m. There had been plenty of opportunity to select clear
bearing points. But we were novice students in navigation. The most common
error was to have calculated bearings in advance which were difficult to
sight once on the water. It was the GPS, which was likely to have been
correct. However, we did receive feedback that GPS could be quite badly in
error when satellites were low on the horizon and cliffs affected the
signal. I'ld be interested to know if anyone has experienced this?

I personally learnt a great deal, much of which could not be found in books
and most importantly the absolute necessity of practising theory in "on the
water" exercises.

And perhaps one day, when their is peace in the world, and yellow finned
tuna jump from the sea into our frying pans, and Mirages, Pittaraks,
Mariners and Nadgees paddle together in contented synchronicity, perhaps
then we will find the elusive Point B!

All the best, PeterO
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