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Cellular forums Home > Archive > Garmin GPS > June 2005 > Odometer accuracy
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| Tommy Hayes 2005-06-13, 6:55 am |
| Hello all,
I've been messing with a newly bought GPS unit, a Geko 201. Everything
seems pretty intuitive, except for the 'accuracy' number. I'm hoping
someone here can explain to me what it means:
I understand that when read an absolute position from the unit and it
tells me 'Accuracy: 10m' then I could be anywhere within a 10 metre
radius of the position reported by the unit, or at least that's the way
I assume it to be.
However, what does this 10m accuracy mean for other types of readings,
for example the trip odometer? Does it mean there will be a 10m
accuracy for the length of the whole trip (so the longer the trip, the
less the relative error). Or something different (maybe the both the
start point and the end point have a 10m accuracy, giving a combined of
20m for the whole trip?) Or is the unit entirely free from the
inaccuracy it has when trying to determine absolute position?
Related, is the trip odometer any good on a handheld GPS unit? I
understand that it measures the total distance travelled by the unit.
If I carry the unit in my hand, and my natural walking habit is to
swing my arms wildly (to take an extreme case), will that greatly
increase the total distance travelled by the unit, or does it not pick
up such 'micro' movements (the swing of my arm being less than the
advertised accuracy of the unit).
Thanks,
Tommy.
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| Tom H. 2005-06-13, 10:55 pm |
| Tommy,
The accuracy number refers to the accuracy for a single position fix. AFAIK
the manufactures do not really give an exact definition of the accuracy they
report, but from things I have read by others, and my own sense of it, it is
probably the accuracy that can be expected based on satellite geometry
(where the sats are in the sky relative to your position). Other sources
of inaccuracy are probably not taken into account, and the manufactures do
not specify a confidence level (i.e. is there a 50% probability of being
within 10 meters or 95% probability etc.) So that figure should only be
used as guideline. The largest error you are likely to encounter is
something called multipath: a situation in which one or more of the
satellite signals is being reflected off an object before being received by
the GPS receiver.
I also have a Geko 201, and the accuracy of the trip odometer seems pretty
good to me. I have compared its readings to known distances on topo maps
and to my bike odometer. I calibrated the bike odometer by taking an actual
measurement of the tire circumference. On a recent ride my bike computer
read 11.82 and the Geko 11.7 miles, about a 1% difference.
Tom
"Tommy Hayes" <tommy.hayes@gmail.com> wrote in message
news:1118651780.446323.235410@o13g2000cwo.googlegroups.com...
quote:
> Hello all,
>
> I've been messing with a newly bought GPS unit, a Geko 201. Everything
> seems pretty intuitive, except for the 'accuracy' number. I'm hoping
> someone here can explain to me what it means:
>
> I understand that when read an absolute position from the unit and it
> tells me 'Accuracy: 10m' then I could be anywhere within a 10 metre
> radius of the position reported by the unit, or at least that's the way
> I assume it to be.
>
> However, what does this 10m accuracy mean for other types of readings,
> for example the trip odometer? Does it mean there will be a 10m
> accuracy for the length of the whole trip (so the longer the trip, the
> less the relative error). Or something different (maybe the both the
> start point and the end point have a 10m accuracy, giving a combined of
> 20m for the whole trip?) Or is the unit entirely free from the
> inaccuracy it has when trying to determine absolute position?
>
> Related, is the trip odometer any good on a handheld GPS unit? I
> understand that it measures the total distance travelled by the unit.
> If I carry the unit in my hand, and my natural walking habit is to
> swing my arms wildly (to take an extreme case), will that greatly
> increase the total distance travelled by the unit, or does it not pick
> up such 'micro' movements (the swing of my arm being less than the
> advertised accuracy of the unit).
>
> Thanks,
> Tommy.
>
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