[OSM-dev] Overlapping polygons - verboten or not?

[Ben Supnik]

Hi Josef,

Slight aside...your questions about "is this really allowed" mirror my 
thinking when I first started working with OSM data for X-Plane.

The very question "is X permitted" is a somewhat problematic one for 
OSM.  I would almost compare it to the question "are misspelled words 
allowed on Wikipedia"?  I think the Wikipedia editing standards probably 
frown on misspellings, but at any give time, between when a user 
misspells a word and when another user fixes it, a snapshot of 'the 
wiki' would reveal violations of the policy.

OSM takes this to the next level: not only is there the risk of 'policy' 
being violated at any given time (e.g. there is no enforcement of policy 
when a change is applied) but the very policy itself is a lot looser at 
best.  I don't know how much of this is culture (I have seen a share of 
opinions on this list that the wiki isn't particularly authoritative), 
how much is technical (the people 'spec'ing things on the wiki don't 
always maintain the code, so you can write a spec for a super-way and 
nothing will happen) and how much is process (since we have no 
super-strong policies, client code takes a fuzzy approach to data, which 
makes policy less important, repeat loop) but...either way, you can't 
really view the wiki as a definitive guide to the data.

So...if you want to actually _use_ the data, what can you do?  Once you 
clear your mind of old-school GIS-like ideas, things will be easier. :-) 
  In X-Plane's case, we...

1. Attempt to align our interpretation of the data as much with the maps 
as possible.  That is, if data seems broken in the .osm file but the 
maps render it, I figure I need to figure out their fall-back case and 
implement it, so that my users don't have to worry about cleaning data 
in ways they can't see and fix.

2. Process the data with a really robust set of tools, so that we don't 
depend on any geometric properties of the data.  We use CGAL, which 
provides robust polygon cutting under pretty much any conditions, no 
floating point limits to worry about.  We explicitly check for 
degeneracies and decide how to handle them (e.g. what if a polygon is 
self-intersecting, empty, holes go through the outer boundary, etc.).

3. We sanity check the incoming data.  This is fairly specific to our 
app case and may not be appropriate for others.  If a way goes straight 
up a mountain at a 80 degree slope and its type is motorway=trunk, we 
throw it out.  It's much more likely that there's a conflict between our 
elevation data and the road than that there's a road going up at that 
slope, and frankly it's going to look stupid in X-Plane anyway.  So we 
protect ourselves from data problems by being more committed to 
'plausibility' than 'accuracy'.

But...a few rays of hope in all this...

- A traditional GIS db might tell you the problems, but it will _still_ 
have a ton of problems.  (Heck, try VMAP0, it'll have problems they 
declare and problems they let you discover. :-)

- In practice, the value proposition of OSM makes it worth it.  The data 
quality and coverage world-wide is pretty unprecedented...

I'm not sure if that answered your question, but hopefully it sheds some 
light on why there aren't really good answers to your questions.

cheers
ben

On 4/20/11 12:11 AM, the.promenader at gmail.com wrote:
> A simple question:
>
> Can someone point me to an explanation/discussion about whether overlapping polygons are 'permitted' or not? I get a different answer every place I look, and I need to get this question out of the way before before I can even consider developing with OSM.
>
> Cheers,
>
> Josef.
> _______________________________________________
> dev mailing list
> dev at openstreetmap.org
> http://lists.openstreetmap.org/listinfo/dev
>

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