[Talk-ca] Multipolygon problems

Jochen Topf jochen at remote.org
Fri Jun 30 19:21:28 UTC 2017

On Fri, Jun 30, 2017 at 08:16:40PM +0200, Frank Steggink wrote:
> Maybe I'm not understanding it, but in the OSM inspector [1] I just see one
> case of old style multipolygon, in Manitoba. Last week, when you posted your
> original message, I just saw one case in New Brunswick. IIRC, it was a park,
> not even from the Canvec import.

The types of problems I am talking about don't show up in the OSM
inspector. This is not old-style multipolygons (where tags are on the
outer ways and not on the relation), but multipolygons where the tags
are on the relation AND on the ways.

> In the OSM inspector other errors can be seen, but the most prevalent one is
> "Touching rings". Maybe indeed a case of suboptimal mapping, but nothing
> which seems urgent to me.
> Here is an example of a forest multipolygon, imported by me
> (canvec_fsteggink). It is still version 1, but it has tags on the relation,
> not on the rings (except for the quarries): [2]
> This is from Canvec v7.0. IIRC, we started at v6.0, and the last version I
> know of is v10.0. Maybe v6.0 had wrong tagging, but I'm not seeing any such
> cases in the OSM inspector.
> So, I'd like to ask you to give a couple of examples where data imported
> from Canvec is clearly wrong with regard to old style multipolygon tagging.

Here are all cases in Canada (not only those from the imports):

Here is one example where you can clearly see the problem:

> When we have clear examples, then it might be easier to come up with a plan
> how to fix it. But so far, I see absolutely no reason why Canada stands out
> in a negative way. Yes, we all acknowledge that Canvec data is suboptimal,
> but as others already have pointed out, mapping everything by hand in
> especially remote areas is nearly impossible.

Canada stands out in a negative way, because
a) there are so many problems. Nearly a third of the cases worldwide are in
   Canada and
b) most of these problems are probably caused by one little program, the
   program used to convert/import the CanVec data.

Mapping Canada "by hand" might be difficult because it is such a huge
country and there aren't that many mappers. But the same arguments goes
for why you have to be extra careful importing data. If you break
something, there are not enough people to fix it manually. And, yes,
errors do happen. And if we find them, we fix them and move on. But
errors from imports can be so huge there aren't enough people there to
fix them manually. So I think it is the job of those who did the import
in the first place, to fix their work. If you add data to OSM you take
on a certain responsibility. If you add more data, you have a larger
responsibility. But saying: We don't have the manpower, so we are taking
a shortcut and then, when it turns out the shortcut wasn't so short
after all, whining that you don't have the manpower to fix it. That
can't be the excuse.

Jochen Topf  jochen at remote.org  https://www.jochentopf.com/  +49-351-31778688

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