[Paddlewise] GPS Errors Summary

G'Day,

Thought it would be worth summarising those posts in the GPS thread which
discussed GPS and coastal navigation errors. This is not to argue against
GPS which most contributors considered a very useful tool. Nevertheless the
comments seemed to be useful in describing how occasionally errors might
occur. Hope I don't misinterpret people: -

1. Peter Rathmann, during six years daily experience of using a GPS in a
variety of activities, occasionally observed errors so large as to be
obviously wrong, even though they did not show up as exceptionally high
estimated position error or accuracy indications. He ascribed these either
to momentary multi-path reflections off buildings or to the GPS receiving
signal from only three satellites.

2. Mike Daly reported that his GPS frequently shows an EPE of less that 15m
and he would ignore errors at that low level though larger errors he would
take notice of. Mike also mentioned that if only three satellites were
visible, then the receiver would assume the current altitude based on the
most recent determination. This gave rise to a discussion on how changes in
altitude could give rise to errors when moving from land to sea or paddling
in lakes.

6. Bruce Grubbs reported errors of several miles in saved way points when he
had a three point fix only (Bruce is a pilot and didn't say whether the
error was while flying or paddling).

3. My own experience with a GPS during a coastal navigation exercise
comparing a GPS against dead reckoning and non GPS pilotage. Almost
certainly the GPS error was negligible, and our non GPS measurements had an
average 10% error i.e 900metres over about 10km.

4. A second hand report from a yacht in Jervis Bay of a GPS reading that
computed their position to be on a reef, which they were nowhere near. The
explanation given was a possibility of interference by a cliff in the signal
transmission path because the satelites were near the horizon.

5. A report back channel that if the chart co-ordinates were supplied in a
different datum to the GPS then significant errors could occur. I don't
completely understand this but it seems as if it could easily be checked
before going on a trip.

The most common source of errors seemed to be when only three satellites
could be received. Those who reported occasional errors nevertheless felt
the GPS to be a very useful tool.

Hope this is helpful,

All the best, PeterO
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