Kinky Cowboy wrote:
> On Thu, 08 Nov 2007 23:04:19 GMT, Newbie FMP User <[email protected]>
> 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/