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[SomeoneElse]

On 30/01/2015 14:12, St Niklaas wrote:
> > From: François Lacombe <fl.infosreseaux at gmail.com>
> >
> > Since OSM editing tools aren't AutoCAD you can't be 100% precise on the
> > geometry.

Exactly so.

>
> Francois if you’re using JOSM you’re be able to work up till 0,06 - 
> 0,04 =0,02 m accuracy
>

No.  Unless you can measure accuracy on the ground to that level of 
precision, you simply can't*.  The imagery sources availble to OSM 
aren't that accurate, and even the aggregated traces of many, many 
consumer GPS units won't hit that accuracy.  0.06m is a tiny amount 
compared to the amount that natural processes can cause a particular 
"location" to move**.

A good rule of thumb for OSM is "don't try and map more accurately than 
your sources".  If you only have aerial imagery, or only have a few GPS 
traces, don't try and map every last hedgerow, since you simply don't 
know how accurate the sources that you're working from are.  Instead, go 
out and collect more data.  For example, once you know how aerial 
imagery compares to lots of GPS traces (and vice-versa - GPS traces can 
have a systematic offset due to terrain and even "what side of a road 
people are allowed to walk down") you're in a much better position to 
contribute.

Cheers,

Andy

PS:  Please don't reply to digest posts largely unedited.  The very 
first line in your reply mail was, quoted, "When replying, please edit 
your Subject line so it is more specific".  Even if you do edit a digest 
post down it'll still destroy the threading of the mailing list.  Many, 
many people rely on this to make the process of reading lists such as 
"tagging" less of a chore (it's much easier to "mark a thread as read" 
than it is to go through every post about semicolons, patron saints or 
pipelines one by one).  If you don't have a mail reader than can handle 
non-digest mail, you probably need to change the way that you read mail.

* The exception is where you're triangulating from points measured with 
an accuracy beyond what consumer kit can provide.  I'm looking forward 
to seeing "triangulation via theodolite from X" in an OSM source tag :)

** http://www.bbc.co.uk/news/science-environment-12732335
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