[Talk-ca] Multipolygon problems

Frank Steggink steggink at steggink.org
Fri Jun 30 21:47:36 UTC 2017


On 30-06-2017 21:21, Jochen Topf wrote:
> 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.
Ah, ok, now I understand. Since there was a lot of discussion about old 
style multipolygon tagging, and since this type of problem hasn't been 
added to OSM inspector,  this wasn't immediately obvious.
>> 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):
> https://tmp.jochentopf.com/954226a3acab882d28d8500ddef8203d/same-tags-ca.pbf
>
> Here is one example where you can clearly see the problem:
> http://www.openstreetmap.org/relation/541821
How difficult would it be to add this to OSM inspector? Not everybody 
has Postgres running, and is able to use osm2pgsql. Yes, there is 
documentation, but it requires some technical skills. Also, it would be 
very convenient to have this updated daily.
>> 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.
As you might have noticed, later imports, like the example I provided, 
don't have that issue anymore. I'm mentioning this to express that not 
_all_ Canvec data is at fault! Only the first couple of versions. 
However, for some reason this was never noticed up until a point that 
collaborative action was done to have it fixed. Probably because the 
rendering pipeline of the slippy map was accepting this kind of tagging 
up until recently.
> 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.
The world is so huge that there aren't enough people to create and 
maintain a global world map. However, OSM exists. Fixing errors can also 
be crowdsourced. Martijn van Exel is really doing a great job with 
MapRoulette, for instance. Although fixing errors (cleaning up the mess 
left behind by others) is not nearly as rewarding as mapping, it might 
be easier to do, especially since there is no need for a lot of 
creativity when fixing the same kind of errors.
> 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.
The person who did most work initially on the Canvec import has already 
left OSM about five years ago. This was during the license change. He 
joined one of the forks, from which we hear nothing nowadays. So, don't 
count on him, and possibly not on others who were working on the Canvec 
import at that time. I'm sure he and others who were involved at that 
time regret certain decisions being made and actions being done.

However, the import was supported by the majority of the community at 
that time, and although there are people who have criticized the import 
(and also of the Geobase roads) they still exist in OSM and are 
gradually being improved by others.
> 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.
I'm not using it as an excuse, but as a fact. I don't know how 
"complete" the Canadian map is, but I'm sure that it will be way less 
complete when imports wouldn't have been done. I recall that one day 
I've driven about 800 kms, for about 12 hours in total, but the 
resulting GPX file doesn't look that "impressive". It barely made a 
dent, even at that time... Just a couple of main roads and some 
adjustments in the database. The same applies to digitizing, or as it is 
also called, "armchair mapping".

Personally, I think that, although things were far from perfect, they 
were done with the best intentions and with the support of the majority 
of the Canadian OSM community. We have to deal with this situation now. 
A much more cooperative tone would have been very welcome, especially 
since you would like to see us coming off our lazy butts and fix our mess.

There is really nothing to gain by threatening to contact the DWG in 
order to have those imports removed. They already exist for about 7 
years! And if the Canadian community at large wouldn't have welcomed it, 
this would have come to the surface way sooner.  So, why has this 
suddenly become such a huge problem because of the way how the slippy 
map is rendered?

We can much better focus on getting the job done, than criticizing each 
other. If 150 people are fixing 100 multipolygons each, this is doable! 
We could do it with the help of OSM inspector, and eventually a 
MapRoulette task.

> Jochen
Frank

p.s. Are you still wearing your t-shirt with Lake Manicouagan on it, 
based on OSM data? I hope it doesn't contain wrong tagging or imported 
data. ;)



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