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Date: 08 Nov 2007 23:04:19
From: Newbie FMP User
Subject: accuracy of Garmin Edge 305
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Does anyone know the accuracy of this GPS unit, particularly for
altitude?
Thanks
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Date: 09 Nov 2007 16:54:41
From: Booker Bense
Subject: Re: accuracy of Garmin Edge 305
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In article <new-3A7586.15043508112007@newsclstr02.news.prodigy.com >,
Newbie FMP User <new@publishing.net > wrote:
>Does anyone know the accuracy of this GPS unit, particularly for
>altitude?
>
For altitude it's probably as good as any other wristwatch sized
barometer, it does a reasonable job of using the GPS to keep
itself calibrated. Upgrading the software to the latest version
seems to help quite a bit. In terms of relative alititude change
it's quite good[1], the absolute value is only as good as the last
calibration.
In my experience it doesn't like really cold weather much, in
below freezing temps you need to keep it warm.
_ Booker C. Bense
[1]_ Or at least consistant. If you do the same loop/climb you
generally get the same altitude gain within a few percent at
most.
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Date: 09 Nov 2007 00:43:53
From: Kinky Cowboy
Subject: Re: accuracy of Garmin Edge 305
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On Thu, 08 Nov 2007 23:04:19 GMT, Newbie FMP User <new@publishing.net >
wrote:
>Does anyone know the accuracy of this GPS unit, particularly for
>altitude?
>
>Thanks
It has a barometric altimeter, which is about as accurate as any other
barometric altimeter - i.e. not that good if the weather changes. GPS
altimetry is even worse, which is presumably why Garmin chose to use a
barometric unit.
Over a relatively short time of unchanging weather, the barometric
unit on my Polar is sensitive enough to differentiate between the
bottom and top of the velodrome banking, but from one week to the next
it gives absolute errors of hundreds of feet. The Garmin unit is
similar. GPS altimeter error is about +/- 70 feet on a good day, so
absolute error is smaller but resolution is much coarser.
Kinky Cowboy*
*Batteries not included
May contain traces of nuts
Your milage may vary
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Date: 09 Nov 2007 09:24:59
From: Nigel Cliffe
Subject: Re: accuracy of Garmin Edge 305
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Kinky Cowboy wrote:
> On Thu, 08 Nov 2007 23:04:19 GMT, Newbie FMP User <new@publishing.net>
> wrote:
>
>> Does anyone know the accuracy of this GPS unit, particularly for
>> altitude?
>>
>> Thanks
>
> It has a barometric altimeter, which is about as accurate as any other
> barometric altimeter - i.e. not that good if the weather changes. GPS
> altimetry is even worse, which is presumably why Garmin chose to use a
> barometric unit.
>
> Over a relatively short time of unchanging weather, the barometric
> unit on my Polar is sensitive enough to differentiate between the
> bottom and top of the velodrome banking, but from one week to the next
> it gives absolute errors of hundreds of feet. The Garmin unit is
> similar. GPS altimeter error is about +/- 70 feet on a good day, so
> absolute error is smaller but resolution is much coarser.
Garmin are actually quite smart with the combination of barometric and GPS
altitudes(*).
As KC says, both have problems; GPS is not particularly accurate (though I
find it better than +/-70ft vertically) and barometric has to be
recalibrated regularly to known reference points to maintain accuracy.
Garmin use the GPS data to auto-correct the barometric. So, if you forget
to calibrate the barometric, the GPS keeps the value within sensible bounds.
It works surprisingly well; I can leave the unit uncalibrated for over a
week, look at the data tracks for a week walking mountains in the NW of
Scotland (which can have weather causing barometric changes of hundreds of
metres in a few hours) and cross-check against the map height data. Its
usually well within 10m, mostly within 5m.
I have known it go wrong; I was on one hill last year with the height
reading 100m over reality. No amount of "recalibrate" would help. Ended up
with power-off and power-on cycle to sort it out and it then remained fine
for rest of day. Proof that one should not rely totally on instruments and
always have a clue about "valid" readings.
That said, I am not a fan of the Edge series devices; they cannot output the
current position in any map grid system. So, if lost one is unable to
cross-check the GPS coordinates with a paper map. I guess if you plan to
link it to a training programme, they have their uses, but as a navigation
device, I would look elsewhere (including others in the Garmin range).
(* at least on their hand-helds, such as Etrex, Geko, 60-series, etc)
- Nigel
--
Nigel Cliffe,
Webmaster at http://www.2mm.org.uk/
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Date: 09 Nov 2007 11:07:26
From: Lou Holtman
Subject: Re: accuracy of Garmin Edge 305
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"Nigel Cliffe" <me@privacy.net > wrote in message
news:fh1919$phn$1$8302bc10@news.demon.co.uk...
>
> That said, I am not a fan of the Edge series devices; they cannot output
the
> current position in any map grid system. So, if lost one is unable to
> cross-check the GPS coordinates with a paper map.
If you want to know your current position in coordinates, mark your position
as a waypoint and the coordinates are shown so you can crosscheck them on a
map.
Lou
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Date: 10 Nov 2007 18:43:09
From: Testing FMP
Subject: Re: accuracy of Garmin Edge 305
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Thanks to everyone for some very useful knowledge.
To finish off the altitude question, I wonder if a user can manually
calibrate the altimeter from a map, or whether the GPS is the only
calibration?
Now I hope you will bear with me while I get slightly obsessive. Now
I'm wondering how accurate the Garmin is for the measurement of distance
travelled.
Does the Garmin use the wheel speed sensor (when available) for distance
only when it loses the satellites (in a tunnel, etc) or to continuously
correct/calibrate the GPS measurement?
Let's assume there is no wheel sensor. How accurate is the GPS measured
distance if you are moving on a level surface? What would it measure if
taken around a 1/4 mile oval track for a given number of laps? Or, to
get even more ridiculous, how small would the track have to be before
Garmin decided it wasn't moving?
Thanks
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Date: 10 Nov 2007 20:52:00
From: Lou Holtman
Subject: Re: accuracy of Garmin Edge 305
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Testing FMP wrote:
> Thanks to everyone for some very useful knowledge.
>
> To finish off the altitude question, I wonder if a user can manually
> calibrate the altimeter from a map, or whether the GPS is the only
> calibration?
You can't calibrate it manually. It is not necessary. My experience is,
if you turn on the unit and wait for it to lock on the satellites the
altitude that is shown is not correct. If you wait a couple of minutes
you see the altitude changing to the 'correct' value within 5- 10
meters. After that it is amazingly accurate.
>
> Now I hope you will bear with me while I get slightly obsessive. Now
> I'm wondering how accurate the Garmin is for the measurement of distance
> travelled.
The measured distance is OK. The current speed is very jumpy.
>
> Does the Garmin use the wheel speed sensor (when available) for distance
> only when it loses the satellites (in a tunnel, etc)
Yes. I can't understand why they do that.
> or to continuously
> correct/calibrate the GPS measurement?
No.
>
> Let's assume there is no wheel sensor. How accurate is the GPS measured
> distance if you are moving on a level surface? What would it measure if
> taken around a 1/4 mile oval track for a given number of laps? Or, to
> get even more ridiculous, how small would the track have to be before
> Garmin decided it wasn't moving?
You can turn off the GPS. After that the unit uses the wheelsensor.
Don't be obsessive about it. It is my experience that the measured
distance is as accurate as a wheelsensor.
Lou
--
Posted by news://news.nb.nu (http://www.nb.nu)
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