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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/
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