[OSM-talk] [BOT] [RFC]: water surfaces

[Andy Townsend]

On 22/03/2016 13:10, Frank Villaro-Dixon wrote:
> Hi everybody,
>
>
> So, what do you think ?

I think it's a silly idea.

Identifying complex potentially problem areas is one thing - as you've 
found, attempting to fix them automatically is quite another.  In among 
the "obvious" fixes will be many harder to find new problems that you 
have introduced.


>
> Technically, it was already run on the whole planet, and so far no 
> bugs were found. 

That's not true.  Many people complained and all your work was reverted.*


> Now, I need your comments and/or your approval, critiques, etc.
> Tell me what you think ;-)
>

Here's what I think you should do, when you detect a potential problem:

1) Firstly, before fixing anything, try and understand what the cause 
was.  Perhaps an inexperienced mapper has edited some existing data that 
broke something that they didn't understand?  You'll need to look at the 
mappers who have contributed to the problem, their relative experience, 
and what editors they are using (for example, an iD user may been not 
have seen the complicated reationship between multipoloygons, and a JOSM 
user may have stopped thinking about real-world data and thought _only_ 
in terms of multipolygons - both can cause errors).

2) If you can, go and actually survey the area.  No, really, do actually 
go there.  That way you'll get a full 3d picture in your head of what's 
there and how it relates to the aerial imagery.  It also enables you to 
recognise features from imagery better, so you can see what sort of 
surface a path is, and (with water features) tell man-made ones from 
natural rivers and streams (difficult from imagery, especially when made 
by man 200 years ago).  Maybe the area is inaccessible to everyone, in 
which case anyone would have to work from imagery and other out of 
copyright sources, but if it is accessible to local mappers then they 
are the best people to fix any problem because they will be able to do a 
proper survey.

3) You'll now have a picture of (a) what the original mapper had in mind 
when they mapped it, (b) what subsequent mappers were trying to do and 
(c) what you'd have mapped it as, if you had mapped it from scratch.

If these three all agree, and it was just a tagging error (for example 
I've seen people add "natural=foo" instead of "name=foo" recently) then 
it makes sense to "just correct the data".  However, it's quite likely 
that these three might disagree, and perhaps you need to explain to an 
earlier mapper how multipolygons work, or to someone who has come along 
and "corrected" data in the interim that what they've changed something 
to is a valid OSM tag, but doesn't actually match what's on the ground 
in this case.

The best way to try and communicate with a specific previous mapper is 
via a changeset discussion comment.  The advantages of doing it this way 
are that the discussion is public, and the context is obvious, as it's 
visible with the changeset.  Other local mappers can also add comments 
there too - perhaps someone else locally has more knowledge about a 
particular water body.  If that doesn't work, or you need to contact all 
local mappers you can try adding an OSM note explaining the issue.  This 
might not get picked up immediately but notes sometimes do get fixed 
many months after they were added. Another option is to try and contact 
local users via a country's mailing list, forum or IRC channel.  They 
may know someone who is local, or know someone who knows someone (not 
necessarily an OSMer) who may be able to answer questions.


Based on the above, I don't believe that it is possible to do the sort 
of "water fixup" that you were trying to do entirely automatically. 
http://wiki.openstreetmap.org/wiki/Automated_Edits_code_of_conduct has 
sections that require you to "document and discuss your plans" for a 
reason.  To take a specific example, I noticed local edits made by you 
in http://www.openstreetmap.org/changeset/37086092 that simply failed to 
understand what the original mapper was trying to represent.  I pointed 
this out on the changeset discussion, and was frankly amazed when you 
created a bot account to make more of exactly the same sort of error, 
again.  Maybe I'm the frog in 
https://en.wikipedia.org/wiki/The_Scorpion_and_the_Frog :)

There may be many scenarios that you haven't considered when designed 
what automatic changes you are trying to make.  Other mappers will be 
able to help you understand those when you discuss your plans with them.

Also, please don't think that "changing a tag to one that is valid 
within OSM" means "making the data correct" - it doesn't.  All it means 
is that it is no longer possible to automatically find potentially 
problematical areas needing survey, or find mappers who may need help to 
map better.  In an analogy, if someone has described a "horse" as a 
"kow" correcting the spelling to "cow" does not make the description 
correct.

Finally, please remember - OSM is about geography, not computer 
science.  Your account has made relatively few edits and few if any of 
these seem to be based on actual survey.  I would strongly suggest that 
you take a little time out to actually do some real survey-based 
mapping, and in addition spend a bit of time understanding the human 
causes of the sorts of problems that you're aiming to detect, and 
helping those people understand the resulting problems in the data.  
Don't just say "you did X wrong" - explain to them politely how and 
offer to help them get it right next time.

Best Regards,

Andy Townsend (SomeoneElse)

* as well as being an "ordinary mapper", I'm a member of the Data 
Working Group and saw the discussions as these changes were made and 
reverted.