[Talk-ca] What's up with those forests in Canada section

James james2432 at gmail.com
Thu Sep 1 02:40:54 UTC 2016


+1 a lot more detailed than what I wrote

On Aug 31, 2016 10:26 PM, "Sam Dyck" <samueldyck at gmail.com> wrote:

> Here's my suggestion for a sort of FAQ (in wiki markup), incorporating
> what James already wrote. I'm posting it here for comment because I have a
> tendency to get unhelpfully passive aggressive.
>
> The squared off sections of forest in Canada are the result of unfinished
> CanVec data import. CanVec tiles are broken up into squares called the NTS
> grid to better manage the data. If you see a forest that's squared off with
> a empty section beside it, it's most likely that that grid has not been
> imported yet.
>
> ''What is Canvec?''
>
> [[Canvec]] is a digital product produced by the federal government that is
> a combination of various federal geodata databases into 1:50000 tiles.
> These tiles were converted by Natural Resources Canada into OSM XML and put
> on a government FTP server for importation into OSM. After several years of
> licensing discussion.
>
> ''Some of the data in a Canvec import changeset has something weird going
> on (forests overlapping in lakes, islands where there don't appear to
> islands, wetlands where there sohuld be lakes). Why are you importing this
> garbage?''
>
> Canvec is generally accurate, it was collected from high quality satellite
> imagery collected for the federal government, and has generally withstood
> our attempts to ground truth it. However there are errors and apparent
> errors. Some of these can be explained by natural changes: lakeshores shift
> with the years and seasons, lakes become wetlands, forests burn or are cut
> down and regrow.
>
> The simple reason we have to do this import is because Canada is enormous
> and has very few people, consequently there are large areas that have a
> very light human presence. For example the territory of Nunavut, the
> largest subnational division in Canada, is larger than of France, Ukraine,
> Sweden and the United Kingdom combined and has less than 40,000 people.
> Most people in Canada live in a handful of cities a short distance from the
> US border. There is a lot of blank area to fill, and so we make an effort
> to import quality data, but there is a lot of area to cover, so after long
> discussions we arrived at the consensus that importing Canvec data was the
> best solution, providing we followed a set of practices.
>
> ''Don't you have local mappers in these communities who could check the
> data?''
>
> Most likely no. See the note about population density above. Also much of
> non-urban Canada, especially Northern communities, have to rely on
> satellite internet, which is both extremely expensive and has both
> effective download speeds measured in kbps and small data caps of 5 or 10
> GB.
>
> ''I see some issues with Canvec data, what should I do?''
>
> If you think the data itself is in error, try and check to see if it could
> not possibly be an accurate reflection of what might be at some point.
> Canvec importers have been criticized for importing data, that while it
> looks suspicious, accurately reflects what is on the ground. If it's an
> obvious error that's easy to fix, go ahead and correct it. If there's
> something bigger, talk to the mapper or post on the talk-ca mailing list.
>
> ''I see something wrong with the actual structure of the data (overly
> complex ways, duplicate ways).''
>
> These should have been fixed in the import, but sometimes things get
> missed. Please go ahead and fix them.
>
> ''I found a Canvec import that didn't comply with the import policy!''
>
> Please don't revert it, despite the appearance of wholesale importing, a
> proper Canvec import takes a lot of time and effort on the part of the
> importer. Canvec imports began before the current import policy, and so
> some importers continued what they had already been doing unaware of the
> policy. Hopefully everyone is in compliance now, but if you do see
> importing incorrectly please assume good faith.
>
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>
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