[Az-Geocaching] Accuracy of GPS's

[Jim Scotti]

Besides bells and whistles, there are some other differences in GPSs that 
affect the accuracy of the units.  Already mentioned is including the WAAS 
data which I think most current units already can do - you can turn it on and 
off in most units.  Some units can also average waypoints now so that you can 
set the unit on a rock or table or whatever, set a mark and then press an 
"Average" software button that will average readings until you stop it.  The 
more measurements, the better the accuracy of the waypoint.  That will 
improve the accuracy of the waypoint given random errors in the whole GPS 
system (there are many systematic errors that WAAS and DGPS can correct for 
that we have no easy way of estimating in real-time).  For example, if you 
watch the track log of a stationary unit, you can see it drifting all over 
the place around your real location as all the variables change, but the 
points cluster around your real location and if you hand averaged all those 
track log points, you'd see that they converge in large numbers on your real 
location.

One other improvement in modern units is improving the precision of the data 
stored.  My old Legend only stored its position internally to a precision of 
maybe a few meters.  Newer units have added some bits to the precision of its 
internal storage and while that precision is beyond what the commercial GPS 
system gives, it might help to smooth out the data, particularly short-term 
trends in the position data either due to errors in the system or motion of 
the unit.  I think the Legend/Vista C and Cx as well as the Garmin GPSMap 
60C(x) for example have the improved storage precision.  You might only see 
the difference if you plot your track logs with a map scale of a few 10s of 
feet, maybe comparing an older and newer unit.

Finally, the accuracy does depend on the satellite reception, so a unit that 
can acquire and track more satellites with better signal will do a better job 
of measuring your position.  On a hike last year I carried my old (less than 
trusty, poor old thing) Legend while my buddy carried his GPSmap 60C.  We 
compared track logs afterwards and I had dropouts all over the place and then 
large uncertainties in other places while his unit kept tracking satellites 
through most of the hike and kept the accuracy up most of the time as well. 
The 60C series has a large antenna and also a better GPS reciever so it can 
track satellites better because of that.

Jim.

On Thu, 7 Sep 2006, joe Highley wrote:

>   Is one gps really more accurate than other gps's?  I have a etrex legend 
> that was about $100 and there were other units for three to four times that 
> amount.  Does more expensive mean more accurate?  Or just more bells and 
> whistles?
> Thanks, Joe.  Just hike.
>
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Jim Scotti
Lunar & Planetary Laboratory
University of Arizona
Tucson, AZ 85721 USA                 http://www.lpl.arizona.edu/~jscotti/