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Cellular forums Home > Archive > Garmin GPS > February 2007 > Precision of gps
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| SissioPai 2007-02-14, 4:33 am |
| Hi everybody.
Reading the specifications of etrex models on the Garmin site, they all give
a precision of 15 meters, without waas. Is it the real precision of these
kind of receivers? I have a cheap bluetooth receiver used with a palm for
road navigation, and when i take points and verify on a map, it seems to me
they are always in a 2-3 meters range from the real position. So, i'd like
to know if that 15 meters of the etrex are real or just the worst case to
write on the specs.
Thanks
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| Alan White 2007-02-14, 4:33 am |
| On Wed, 14 Feb 2007 08:31:37 +0100, "SissioPai" <m@m.it> wrote:
>Reading the specifications of etrex models on the Garmin site, they all give
>a precision of 15 meters,
They give an accuracy of fifteen metres, not a precision of fifteen
metres.
The figure is the accuracy achieved for 95% of the time over a thirty
day period. Sometimes it will be more, sometimes less.
--
Alan White
Twenty-eight miles NW of Glasgow, overlooking Lochs Long and Goil in Argyll, Scotland.
Webcam and weather:- http://windycroft.gt-britain.co.uk/weather
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| BigJim 2007-02-14, 3:33 pm |
| well the way I look at it is you will be in visual range of the target.
"SissioPai" <m@m.it> wrote in message news:45d2b992$1_1@x-privat.org...
> Hi everybody.
>
> Reading the specifications of etrex models on the Garmin site, they all
> give a precision of 15 meters, without waas. Is it the real precision of
> these kind of receivers? I have a cheap bluetooth receiver used with a
> palm for road navigation, and when i take points and verify on a map, it
> seems to me they are always in a 2-3 meters range from the real position.
> So, i'd like to know if that 15 meters of the etrex are real or just the
> worst case to write on the specs.
>
> Thanks
>
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| Nick \(Scots\) 2007-02-14, 3:33 pm |
|
"SissioPai" <m@m.it> wrote in message news:45d2b992$1_1@x-privat.org...
> Hi everybody.
>
> Reading the specifications of etrex models on the Garmin site, they all give a precision
> of 15 meters, without waas. Is it the real precision of these kind of receivers?
I have garmin geko 101, normally accurate within 5m if high on a hill.
Also own a Etrex Cammo, with DGPS on I have had <1m on the top of
'The Caledonian Hill of the Faeries', which is a solitary hill on a remote plain.
I gota sattelite that you are only supposed to get in North America, which I can't
remember the number.
>I have a cheap bluetooth receiver used with a palm for road navigation, and when i take
>points and verify on a map, it seems to me they are always in a 2-3 meters range from the
>real position. So, i'd like to know if that 15 meters of the etrex are real or just the
>worst case to write on the specs.
2-3ms is very good especially if you are moving at the time. Etrex is normlly better
than 15m, unless you are in a tight canyon, close buildings,dense forrest. etc.
Nick
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| Simon Slavin 2007-02-16, 10:33 pm |
| On 14/02/2007, Alan White wrote in message
< pni5t2dgkn7dbuvacs1d
9p3aefsa6hptg3@4ax.com>:
> On Wed, 14 Feb 2007 08:31:37 +0100, "SissioPai" <m@m.it> wrote:
>
>
> They give an accuracy of fifteen metres, not a precision of fifteen
> metres.
Yes ! That's the important thing: not to confuse precision with accuracy.
> The figure is the accuracy achieved for 95% of the time over a thirty
> day period. Sometimes it will be more, sometimes less.
All the modern Garmin units give an accuracy distance on the satellite
page: the say something like 'Accuracy 18 feet'. This is calculated by
using redundancy in what is received from the satellites: if you can get a
3D fix from four satellites, but you have contact with 5 you can use
information from the fifth to check the accuracy of the first four. If it
agrees to within 2 feet you say you have a fix accurate to 2 feet.
Simon.
--
http://www.hearsay.demon.co.uk
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| Alan White 2007-02-17, 4:33 am |
| On Fri, 16 Feb 2007 21:55:36 +0000, Simon Slavin
<slavins.delete.these.four.words@hearsay.demon.co.uk> wrote:
>All the modern Garmin units give an accuracy distance on the satellite
>page: the say something like 'Accuracy 18 feet'. This is calculated by
>using redundancy in what is received from the satellites: if you can get a
>3D fix from four satellites, but you have contact with 5 you can use
>information from the fifth to check the accuracy of the first four. If it
>agrees to within 2 feet you say you have a fix accurate to 2 feet.
Not so.
The displayed figure for accuracy is derived from an algorithm which
GPSr manufacturers keep very close to their chests. In strict accuracy
terms, this figure is objectively meaningless but can be used as a
comparison with other models using the same algorithm.
The only way to determine the accuracy of your GPSr is to position it at
an accurately surveyed point and to compare the position displayed on
the GPSr with the surveyed position of that point. That will give you a
figure for it's accuracy at that instant. If you want a figure for long
term accuracy then you have to average the position at the surveyed
point for a considerable time (30 days?) which not a lot of people are
prepared to do.
As the spec. defines the accuracy as better than 15 metres rms for 95%
of the time over a thirty day period and on a 1:50,000 map 15 metres
converts to 0.3 millimetres, does it really matter?
--
Alan White
Twenty-eight miles NW of Glasgow, overlooking Lochs Long and Goil in Argyll, Scotland.
Webcam and weather:- http://windycroft.gt-britain.co.uk/weather
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| Rudolpho 2007-02-17, 7:33 am |
| Alan White stated the following:
> On Fri, 16 Feb 2007 21:55:36 +0000, Simon Slavin
> <slavins.delete.these.four.words@hearsay.demon.co.uk> wrote:
>
>
> Not so.
>
> The displayed figure for accuracy is derived from an algorithm which
> GPSr manufacturers keep very close to their chests. In strict accuracy
> terms, this figure is objectively meaningless but can be used as a
> comparison with other models using the same algorithm.
Point taken.
> The only way to determine the accuracy of your GPSr is to position it at
> an accurately surveyed point and to compare the position displayed on
> the GPSr with the surveyed position of that point. That will give you a
> figure for it's accuracy at that instant. If you want a figure for long
> term accuracy then you have to average the position at the surveyed
> point for a considerable time (30 days?) which not a lot of people are
> prepared to do.
It can be done quite easily. Mount your GPS on a tripod. Fix that firmly
to the ground. Now let it make a track for as long as you want.
> As the spec. defines the accuracy as better than 15 metres rms for 95%
> of the time over a thirty day period and on a 1:50,000 map 15 metres
> converts to 0.3 millimetres, does it really matter?
On a map no. In real life it matters. In the ravine or close too
--
Rudolpho
"Tip the world over on its side and everything loose will land in Los
Angeles." - Frank Lloyd Wright
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| Alan White 2007-02-17, 7:33 am |
| On Sat, 17 Feb 2007 12:10:05 +0100, Rudolpho <a@b.invalid> wrote:
>On a map no. In real life it matters. In the ravine or close too
What would you do without a GPSr?
--
Alan White
Twenty-eight miles NW of Glasgow, overlooking Lochs Long and Goil in Argyll, Scotland.
Webcam and weather:- http://windycroft.gt-britain.co.uk/weather
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| Nick \(Scots\) 2007-02-17, 3:33 pm |
| > Alan White
> Twenty-eight miles NW of Glasgow, overlooking Lochs Long and Goil in Argyll, Scotland.
>
> Webcam and weather:- http://windycroft.gt-britain.co.uk/weather
Portincaple, Green Kettle ?
Nick
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| Alan White 2007-02-17, 10:33 pm |
| On Sat, 17 Feb 2007 19:25:07 GMT, "Nick \(Scots\)" <nick@nobody.com>
wrote:
>Portincaple, Green Kettle ?
Close :-)
--
Alan White
Twenty-eight miles NW of Glasgow, overlooking Lochs Long and Goil in Argyll, Scotland.
Webcam and weather:- http://windycroft.gt-britain.co.uk/weather
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| Bob Bailey 2007-02-18, 10:33 am |
| Alan White wrote:
> On Sat, 17 Feb 2007 19:25:07 GMT, "Nick \(Scots\)" <nick@nobody.com>
> wrote:
>
>
> Close :-)
>
Seems like, this being a gps ng, you should post lat and long.
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| Rudolpho 2007-02-18, 10:33 am |
| Bob Bailey schreef:
> Alan White wrote:
>
> Seems like, this being a gps ng, you should post lat and long.
You didn't follow the link. I did. This was the result:
Looking West from Whistlefield, Argyll, Scotland.
Twenty-eight miles north-west of Glasgow.
(Ordnance Survey NS236935 or Lat/Long 56° 06' 05"N, 4° 50' 15"W)
--
Rudolpho
Barium: what you do with dead chemists.
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| Bob Bailey 2007-02-18, 12:33 pm |
| Rudolpho wrote:
> Bob Bailey schreef:
>
> You didn't follow the link. I did. This was the result:
>
> Looking West from Whistlefield, Argyll, Scotland.
> Twenty-eight miles north-west of Glasgow.
> (Ordnance Survey NS236935 or Lat/Long 56° 06' 05"N, 4° 50' 15"W)
Ah, humble apologies.
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| Rudolpho 2007-02-19, 10:33 pm |
| Bob Bailey wrote:
> Rudolpho wrote:
>
> Ah, humble apologies.
Apologies accepted :-)
--
Rudolpho
.... GIVE: Support the helpless victims of computer error.
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