[Paddlewise] GPS observation

Hi all

I just thought I'd share a wee observation of GPS use which I found
interesting while paddling a couple of weeks ago.

I was out for a paddle on a very familiar bit of coast by myself, and
planned on heading south down the coast a couple of miles to some skerries,
then off shore east a few kilometres to a buoy, then back to the put in. As
I headed down the coast I noted I was fairly shifting - there was an
appreciable tide running south. There isn't usually much in the way of a
tidal stream here on the East coast (of Scotland), tide sets south turns
sets north, mostly at less than a knot if that, but it was close to a
reasonably large spring tide - which was great as around the skerries I was
heading for is one of the few spots on this coast where the tide
accelerates,  and there are reasonable eddies to play in.

There were a lot of seals around the skerries, so as I set south towards
them I sat watching them (many were sleeping in the water which I rarely
see, not upright but lying horizontally just submerged, eyes closed, big
zzz's. As I approached in the tidal flow I realised the seals, although not
moving a muscle were stationary in the stream! As I pondered how they do
this (they hold their rear flippers at quite an acute angle and look like
they are 'flying' down) I switched on the GPS to work out the speed of the
tide - it was setting south at 3.1 knots around the skerries.

After a while I ferried out to the buoy, up and across the stream. Glorious
day, glassy calm, no wind. Good visibility to the red can now north east of
me.  The  tide eased off away from the headland and shallows around the
skerries and was running at less than 2 knots at the buoy three kilometers
offshore. I sat in the eddy behind it and had a coffee, then thought I'll
head back to the skerries and play about a bit more. I'd logged the outer
skerry as a waypoint as I'd gone past watching the seals, and just for
entertainment (visibility was good) set the GPS on a 'go-to' back to the way
point and left it on the (garmin etrex) compass screen.

The buoy was up stream from the skerries, and as I paddled back into the
tide I lazily paddled across. I then realised that the GPS was pointing me
about 30 or so degrees off course to the skerries. Hmm, checked the waypoint
with the chart, checked the bearing to it with my deck compass, all OK. But
the GPS go-to pointer was 30 odd degrees off. It then myopically dawned on
me that the GPS was lined up with my boat which was pointing more or less
due west, but my direction of travel was considerably south of that because
I was in the tide stream setting south at 2-3 knots. While the GPS knew
where it was and was giving my the right bearing to the skerries, the go-to
pointer is relative to the direction of the movement of the unit, which
because of the set of the tide wasn't the direction my boat and the unit on
deck was pointing. Thus the error in indicated direction!

Paddling across a tide stream where your direction of travel is 'sideways'
is the extreme example, but it follows that if you are making any leeway or
are in an unknown tide stream or eddy, then unless you know which way you
are travelling and line the GPS up with that (unlikely if no transits are
visible) then the garmin compass screen / navigation pointer is going to be
less than useful.

The moral of the story - do not rely on a gps without a steering compass!!

Cheers

Colin

57º19'N  2º10'W


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