[pct-l] GPS, distance and speed.

[dude at fastmail.ca (dude)]

> Do other affordable GPSs do
> pace and distance??

The short answer is "yes".  However, I would caution against relying on 
any GPS for pace and distance because the way that they calculate both 
makes them inherantly inaccurate.  The GPS is not in constant 
communication with the satelites.  It sends/recieves data in 
intervals.  Some GPS devices have longer intervals than others.  About 
the quickest interval I have seen is once per second, but even this 
does not account for times when the GPS device cannot contact the 
satelites for whatever reason (on trails, its usually tree-cover).  The 
GPS only knows where you are when it is in direct communication with 
the satelites, so if there is a gap in communication with the 
satelites, it just assumes a straight line between where you are now 
and where you were at the last satelite communication.  If that was 2 
minutes ago, then it completely omits all the switchbacks and curves in 
the trail that you may have done between now and then, which 
essentially "shortens" the distance you appear to have traveled to the 
GPS device.  This will also make your pace apprear slower.

If your GPS device is one of the really cheap models and only measures 
distance between the "waypoints" that you mannually enter into the 
unit, then this problem because magnified substantially and is VERY 
inaccurate.  The same holds true for pre-entering waypoints using a PC 
and mapping software before your hike.  If you do this, usually the GPS 
device will not have enough memory to hold all the waypoints necessary 
to account for every curve and switch-back in the trail.

OTOH, if you use your GPS device on fairly straight roads and have a 
very clear veiw of the sky at all times (ie: on a highway in a car on 
on a bike), they can be pretty darn accurate.

hope this helps.

peace,
dude

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