[Talk-ca] Primacy of GPS Traces (was Re: Internal CanVec conflicts)

[Connors, Bernie (SNB)]

Tom,

	Differences in GPS results has less to do with the GPS brand and more to do with how the GPS is used, the type of antenna, the satellite geometry, WAAS reception, etc.  If you placed your consumer grade GPS receiver in the exact same location on two different dates you will get two different results.

	It is also important to know that the difference between (N 46 30 00, W 066 30 00) and (N 46 30 01, W 066 30 00) is 31 metres (that's just a difference of 1 second).  A difference of 0.1 seconds is 3 metres.  Most consumer grade GPS receivers only display latitude and longitude to the nearest tenth of a second (0.1") because displaying any additional digits would be beyond the accuracy of the receiver.

	Back in 2003 I did quite a bit of testing with a Garmin GPS 76 and a Garmin 12XL at a high precision control monument.  Every measurement was different.

Best regards,
Bernie.
--
Bernie Connors, P.Eng
Land Information Infrastructure Unit, SNB
bernie.connors at snb.ca



-----Original Message-----
From: Tom Taylor [mailto:tom.taylor.stds at gmail.com] 
Sent: Thursday, 2012-11-15 09:39
To: Harald Kliems
Cc: talk-ca at openstreetmap.org
Subject: [Talk-ca] Primacy of GPS Traces (was Re: Internal CanVec conflicts)

As a geocacher, I'm unhappily aware that different brands of GPS give different results, with differences in the order of 10 meters for an individual point. I suppose a track should be better, since there is an internal consistency check, though not if the difference is due to systemic causes. As it happens, in this case I had my own GPS trace to work with, and it aligned reasonably well with the Bing image of the path I was following.

One thing I'm not sure of: were my GPS traces uploaded with the edits, or did I have to do something special to upload them?

On 15/11/2012 8:20 AM, Harald Kliems wrote:
> Hi Tom:
> welcome to OSM and congratulations on your first edits! Yes, it sounds 
> like you did the right thing. One thing to look out for: Bing images 
> are not always aligned 100% -- they can be offset by several meters.
> Usually this is not a problem but if there are any GPS traces 
> available in your area you (most likely to be found along major roads) 
> you can use those to make sure that the imagery is not off. In JOSM 
> you get the GPS traces by checking the "Raw GPS data" box in the 
> Download window.
> Happy editing,
>   Harald.
>
...

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