Mike Tarburton wrote:
It reports its own accuracy and I am regularly getting 2-4m.
I just want to make sure people don't get the wrong impression from this.
Many people believe that what the GPS unit tells you actually means
something, but this is not really so. Unless you are using differential GPS,
there is no way the GPS unit can possible give you any idea at all of its
accuracy! What it will tell you is what the theoretical limit on your
accuracy is, given the positions of the satellites in use at the time. But
unless this is a very bad figure, it will not mean anything, because
satellite positions are only a tiny factor in determining accuracy in most
situations.
For the most part, it is variations in the atmosphere which control
attainable accuracy. The nature of the atmosphere affects the speed of
light, and that ultimately determines how accurate a fix you get. In
general, the atmospheric effects change relatively slowly, which is why
differential GPS can work - because you can get a correction factor, given
the current state of the atmosphere, which is useful over a broad area.
If you don't believe me, take your GPS for a walk over the same route
several times spaced well apart. Eg an hour apart, or a day apart, or a week
apart. Do it several times. When you plot the tracks over each other, you
will find they wander around over a range far greater than the figure your
GPS gives you. Even if your GPS keeps telling you it is accurate to 5m, you
may still find your paths vary by 50 or 100 metres in places. This will
still look like a small error if you are looking at things at a scale of
kilometres, but don't try to find a Thornbill's nest this way!
Sometimes this inaccuracy can be a real problem. It is actually quite easy
to start down the wrong ridgeline, for example. While your trusty GPS will
eventually give you enough hints that you are in the wrong place, you might
have already dropped a few hundred metres before you realise it!
Cheers, Chris.
Chris Corben
www.hoarybat.com
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