[Qgis-user] wishing for accurate lattitude/longitude from a cell, phone

[Nicolas Cadieux]

Hi Garth,

Good point.  I expect this has more to do with the satellite constellation at the time of the measurement than anything else.  Some reading I have been doing suggest Glonass as being slightly less accurate.  This could be caused by atomic clock precision, or less accurate broadcasting of ephemeris. This could be checked in post processing with a proper GPS.   As I see it, Glonass is Most important when you don’t have enough GPS satellites in view like when working up north or in some settings when satellite are hidden from view (mountain, buildings...).  I don’t think a gps like the eTrex will improve Much if you go from 8 to 16 satellites If the first 8 are well positioned in the sky.

Planning ahead of time to know where and when to measure is always the best practice as I see it.

Nicolas Cadieux
Ça va bien aller!

> Le 2 juin 2020 à 13:56, Garth Fletcher <garth at jacqcad.com> a écrit :
> 
> A QGIS reader pointed me to a Sep 2019 US Forest Service report,
> <https://www.fs.fed.us/database/gps/mtdcrept/accuracy/index.htm>,
> which compares the accuracy of various GPS instruments.
> 
> Their Garmin eTrex 30 section stated that the eTrex's accuracy was ~ 3X
> poorer when using GPS+GLONASS instead of GPS-only mode.  This seemed
> rather counter-intuitive.
> 
> I checked using my eTrex 20 by taking 38 measurements distributed over
> an 8 hour period at a precisely known location, half using GPS only and
> half using GPS+GLONASS modes. Canopy type probably "Light-Medium".
> 
> The results indicate that GPS+GLONASS does increase the accuracy, albeit
> only by a modest amount.  It  most certainly does not decrease it!
> 
> A plot of my results on a 1 meter grid, along with descriptive
> statistics, can be downloaded (69 KB) from:
> <https://smex-ctp.trendmicro.com:443/wis/clicktime/v1/query?url=http%3a%2f%2fwww.Mason%2dNH.org%2fSpecials%2fGARMIN%c2%b1GLONASS%5fTESTS.pdf&umid=73393fec-00d4-4720-895c-3c1b41103d86&auth=276d51547942e21b974d48c8cd7101aa75dd6e1a-edf886b92e985290401859fa5c4c289c0a14c750>
> 
> Cordially,
> -- 
> Garth Fletcher
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