GPS Unit Speed accuracy

For the fun of it, I just put my Garmin 60 CSx in my pocket while I
went skiing. The recorded tracks match perfectly with the satellite
trail images on TopoFusion. I, however, wonder about the accuracy of
the reported speed. It seems very good when I compare it to the auto
speedometer which has always bee on a fairly level road. But, when
I'm on a fairly steep ski slope, does the unti take into account the
change in elevation as well as the horizontal distance in determining
the speed? If I were in an elevator, would it be able to determine
the speed going up or down?

Thanks

I'm pretty sure speed is only 2d speed (although I could very well be
mistaken). Many units also display a vertical speed though which could be
used (or you could probably 'back out' a vertical speed if you have your
track log with elevations).

Dale


Do you have signal in this elevator ;)? If you have a unit with a barometric
altimeter, you should be able to get a reasonable idea (assuming the
pressure in the elevator isn't held constant by some other force).

I haven't tried mine on a ski slope, but when I look at the statistics page,
it tells me that my top speed was 370 mph. I assume that when you're on the
road, the unit adjusts its readings (within reason) to put you on the
nearest road. At times, it has thought that I missed my exit, chided me for
doing so, and then noticed that I instantly teleported over to the correct
road, which probably accounts for the high maximum speed. On a ski slope,
with no road nearby (and the GPS set for pedestrian mode), you probably
wouldn't get that warping effect. Still, if you're having a competition and
betting money on the outcome, it's probably not a good idea to trust the GPS
to give accurate readings.

Just an FYI, most units allow you to turn this option off. Its really only
useful in poor signal environments, so I've never really seen an advantage
to having this turned on. There is also the potential for other things to
happen as far as giving you hiccups in the data collected (probably more
likely than a bug with the lock on road feature). I've seen it happen where
my GPS will put me 1000s of km away, and then bounce me back up to the
correct location in an instant, and then give me the right reading. I think
this has something to do with multipath errors or something like that.


I've been told, that on average the speed readings are accurate to about
0.2m/s. (although I think this is in 2d). This is because the errors in
position between consecutive points are *highly* correlated, so when we
subtract them out, we get a very accurate distance, and hence a very
accurate speed.

Dale