GPS drive tracks are not accurate.

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GPS drive tracks are not accurate.

Mike R-2
From: [hidden email]
[mailto:[hidden email]] On Behalf Of [hidden email]
Sent: Thursday, 5 June 2008 2:38 PM
To: [hidden email]
Subject: [talk-au] Tracing items.

> Having been adding items into osm for the last year I've noticed people
> spending a lot of time tracing roads from landsat, yahoo, etc.

> While this is admirable work is this really a useful exercise.  Someone
> will eventually drive along this road with a gps upload it and then erase
> the traced road.

This is the reason I have stopped wasting time loading roads in to OSM.

I got very keen and loaded lots of roads by tracing them off Yahoo maps,
making sure the trace was an accurate reflection of reality. Then someone
overwrote my plots with a GPS track that was the result of driving these
roads once, with no understanding of the unavoidable inaccuracies in GPS
tracks.

The straight plot of what I knew was straight road was replaced by a wavy
line. I gave up.

Mike






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Re: GPS drive tracks are not accurate.

David Findlay
> I got very keen and loaded lots of roads by tracing them off Yahoo maps,
> making sure the trace was an accurate reflection of reality. Then someone
> overwrote my plots with a GPS track that was the result of driving these
> roads once, with no understanding of the unavoidable inaccuracies in GPS
> tracks.
>
> The straight plot of what I knew was straight road was replaced by a wavy
> line. I gave up.

What about the copyright ramifications of tracing maps? I've done a lot of GPS
tracks and through making multiple passes at various times I've found them
very accurate. Frequently I've found real maps having curves that aren't
there and the GPS is more accurate. It depends on the quality of the GPS fix
and how many times you go over it, if it's just been done once, well chances
are it's could be 10-25m out.

David

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Liz
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Re: GPS drive tracks are not accurate.

Liz
In reply to this post by Mike R-2
On Fri, 6 Jun 2008, Mike R wrote:
> The straight plot of what I knew was straight road was replaced by a wavy
> line. I gave up.

Mike - how did you know it was straight road?
Seeing it on an aerial photo which is not recent may not be good evidence.
If you 'know' the road from recent use, that's reasonable.

I've seen some wavy lines from a direct conversion of a GPS trace (of good
accuracy) to a road.
The wavy bits can be caused by overtaking a vehicle and not considering this
when drawing a road.


The lesson remains that multiple passes with GPS provides best data.

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Re: GPS drive tracks are not accurate.

Kim Hawtin-2
Liz wrote:
> The lesson remains that multiple passes with GPS provides best data.

in my area i've found the ariel photo to be out by as much as 100m.
several GPS passes are with in 10-15m of each other.

my gps is the usb dongle type, and i decided that putting it on
the right hand side of the car yields more consistent traces when
making several trips along the same roads.

for dual dual carriage ways, it probaly doesn't make any difference.
the lanes are so wide that you can sit on the left side of the left
lane, or the right side of the left lane or left or right side of
the right lane and still get data traces that are 20m wide!

cheers,

kim
--
Operating Systems, Services and Operations
Information Technology Services, The University of Adelaide
[hidden email]

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