[Taxacom] Accuracy of GPS Receivers, was...

Piero Delprete piero.delprete at ird.fr
Wed May 8 08:23:07 CDT 2013


Dear all,

as I did a lot of field work in South America, I came to the conclusion that 
measuring altitude with a GPS is totally useless, unless your GPS can 
receive at least 7-8 satellites. This is question of triangulation, because 
the tiniest fraction of an angle (from the satellite to your GPS) means a 
huge difference in altitude. Which is not the case when measuring location.

In conclusion, for altitude, I still use the classic Thommen altimeter, 
which, in my opinion, is more accurate then any GPS. Of course, there are 
variation depending on the anthmospheric pressure, but still more accurate 
the any GPS that capture only a few satellites. Of course, the Thommen 
altimeter needs to be calibrated in a locality that we know the altitude 
with certainty. Living close to sea makes it easier. Or, there are certain 
places where the altitude has been measured with extreme accuracy, which 
could also be a good test for GPS users.

Well, this my two cents,
Piero

Piero G. Delprete - Herbier de Guyane,  IRD - UMR AMAP, Boite Postale 165, 
97323 Cayenne Cedex, Guyane Francaise (French Guiana), France - Tel. [0594] 
0594297250 - http://www.cayenne.ird.fr/aublet2

On Wed, 8 May 2013 05:38:16 -0700
  "Poly, William" <WPoly at calacademy.org> wrote:
>
> GPS accuracy has varied temporally due to restrictions by the US
>govt. for military/security reasons (Selective Availablity).  See:
>
> http://www.gps.gov/systems/gps/performance/accuracy/
> http://www.gps.gov/systems/gps/modernization/sa/
> http://www.colorado.edu/geography/gcraft/notes/gps/gps.html
>
>
> Bill
>
>
> ________________________________________
>From: taxacom-bounces at mailman.nhm.ku.edu
>[taxacom-bounces at mailman.nhm.ku.edu] On Behalf Of Robert Mesibov
>[mesibov at southcom.com.au]
> Sent: Wednesday, May 08, 2013 12:48 AM
> To: TAXACOM
> Subject: Re: [Taxacom] Accuracy of GPS Receivers, was...
>
> Over a month last (austral) summer I took 2 readings a day (total 50
>observations) with my old faithful Garmin eTrex (vintage 2005) from
>exactly the same spot in my backyard, always allowing the readings to
>settle down over the course of a minute or two.
>
> I recorded the date, time, location (UTM), elevation and declared
>accuracy. You can download the raw data here:
>http://www.polydesmida.info/GPS_repeats_Mesibov_2012.csv and use them
>as you like.
>
> As expected, elevation was the most variable data item. I've heard
>it rule-of-thumbed that GPS elevation precision is ca 1.5x horizontal
>precision under the best of conditions.
>
> What's more important to understand is that GPS elevation is height
>above an Earth model, not above sea level, and can be tens of metres
>different from a.s.l. Comparing heights on 10m contour maps with
>heights from Google Earth, I find Google Earth elevations are rarely
>more than 10m off.
> --
> Dr Robert Mesibov
> Honorary Research Associate
> Queen Victoria Museum and Art Gallery, and
> School of Agricultural Science, University of Tasmania
> Home contact: PO Box 101, Penguin, Tasmania, Australia 7316
> Ph: (03) 64371195; 61 3 64371195
>
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Piero G. Delprete - Herbier de Guyane,  IRD - UMR AMAP, Boite Postale 165, 
97323 Cayenne Cedex, Guyane Francaise (French Guiana), France - Tel. [0594] 
0594297250 - http://www.cayenne.ird.fr/aublet2




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