[Talk-ca] Canadian imports: good or bad?

[Steve Singer]

On Wed, 25 Apr 2012, Bégin, Daniel wrote:

> Steve, Paul,
>
> I was on the impression that the consensus was more about using Canvec 
> where it is the best available source and, when it is not, the data could 
> be imported, but only after an exhaustive verification using available 
> data/imagery.

'best available source' as a standard has appeal to me, and I think this 
varies by layer (ie your comments in the other email about older 
hydrography) I think often people are importing all of the layers at 
once when without evaluating what they are importing.

>
> Whatever is the consensus, it should be documented in the wiki.
>

Agreed.  I think right now we have consensus on saying:

* The osm-ca community wants to import Canvec data
* The imports should be done carefully to avoid duplicating objects
* Coastlines and large lakes should only be imported by experienced users

(which is basically what the wiki already says)

Paul proposed two additional guidelines here:
http://lists.openstreetmap.org/pipermail/talk-ca/2012-April/004721.html

"1. The buildings data from CanVec should not be imported unless it can be
verified against imagery, in which case you might as well trace the
buildings from imagery."

Ie if the imagery (and there is no other source like a local mapper) isn't 
good enough to verify the buildings then don't import them. It seems, to me, 
that so many of the 20+ year old building data is no longer valid that we 
might want to discourage the use of this  layer without Do we have consensus 
on this point?


"2. There is not a consensus among the community that CanVec data can be
imported without verifying the data for internal consistency and where
possible against imagery."

I like the sentiment but I don't like the 'negative wording' it doesn't tell 
people what we DO have consensus on, so it doesn't tell them what they can 
import. Nor does it explicitly prevent any sort of import.   My wording from 
this morning apparently wasn't good either.

How about

* When importing Canvec data you should verify that the data you are 
importing is consistent with other data.  For example check that forests 
aren't sitting in lakes. Sometimes the different Canvec layers are not 
consistent because the data comes from different sources.  You should try to 
fix consistency issues as you import data.

(anyone should feel free to propose some better wording)

Is there something we can say in the guidelines to help people judge 
accuracy? (In most of the areas I map I've found the Canvec data lines up 
VERY well with Bing and my GPS traces)


Steve


> Furthermore, I think that "internal consistency/accuracy/existence" should 
> be defined...
>
> consistency: ?
>
> Accuracy: Bing imageries in urban areas are pretty good and easy to 
> correct, if necessary, using available GPS tracks. It is not the case 
> outside these areas.
>
> I suspect that Bing imageries are not always corrected using a good 
> digital elevation model. It means that in hilly areas, the image shows an 
> object somewhere on the ground while the object is actually somewhere 
> else, due to Z distortion.
>
> Existence: Again, outside urban areas, the resolution of Bing imageries 
> doesn't allow for detailed validation. You won't be able to see small 
> objects, even if they are there!
>
> Best regards,
> Daniel
>
> -----Original Message-----
> From: Steve Singer [mailto:steve at ssinger.info]
> Sent: April 25, 2012 07:12
> To: Paul Norman
> Cc: talk-ca at openstreetmap.org
> Subject: Re: [Talk-ca] Canadian imports: good or bad?
>
> On Wed, 25 Apr 2012, Paul Norman wrote:
>
>>> 2. There is not a consensus among the community that CanVec data can
>>> be imported without verifying the data for internal consistency and
>>> where possible against imagery.
>>
>> If no one disagrees with the fact there is not a consensus that
>> importing CanVec without minimal verification is acceptable I'll go
>> ahead and document on the Wiki, using Andrew Allison's examples.
>
> +1.
>
> Is there enough support to use the positive rather than the negative language, ie 'There is consensus among the community that Canvec data should only be imported when the data elements have been verified for internal consistency/accuracy/existence with the available imagery'
>
> Steve
>
>
>>
>>
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