[OSM-talk] Extract a polygon into a polygon filter file format

[Vincent Pottier]

Le 26/08/2011 17:07, Frederik Ramm a écrit :
> Hi,
>
> Phil! Gold wrote:
>> Why is this so logical?  The exact same data can be derived from the
>> intersection of the sets of elements in the two countries' 
>> (single-level)
>> border relations. 
>
> Put yourself in the position of having to create or maintain them.
>
> Creation:
>
> Cascading relations: You create ONE relation that contains all the 
> ways making up the border between A and B (finding them is a 
> cumbersome manual process). Then you create two relations for the two 
> countries, each time adding only 4 or 5 sub-relations.
>
> Single-relation approach: You create TWO relations that each contain 
> all the ways making up the border between A and B (plus a lot more).
>
> Maintenance:
>
> Cascading: You split a way on the border between A and B. The newly 
> created way is added to one relatively small relation and that's it.
>
> Single-relation: The newly created way has to be added to two very 
> large relations which you will have to first download and later upload 
> as a whole.
>
> Single-relation also has the higher risk of edit conflicts.
>
>> It seems to me that because both shcmes can satisfy the
>> use case, the preferable approace would be the simpler one, which I 
>> see as
>> being the single-relation approach.
>
> Simpler for reading, but, as explained above, more 
> difficult/problematic for maintenance.
>
> Bye
> Frederik
>
And if computers are made to ease human work...
So let us give work to computers...

Cascaded relations are already use for borders, hikking routes.
And there is no problem for mapnik that don't care of integrity of 
borders and that don't render routes.

I will also use it for riverbanks of big rivers
*relation type=multipolygon natural=water with members
** role=outer, relation type=multipolygon, riverbank:left_hand with 
members ways waterway=riverbank and waterway=confluence
** role=outer, relation type=multipolygon, riverbank:right_hand ..."..."
** role=inner, relation type=multipolygon, riverbank:isles

that would be better than the actual trunked riverbanks, and riverbanks 
in the middle of the river !
http://www.openstreetmap.org/browse/relation/1705916
The riverbank is strictly a riverbank and can be render with dashed 
line, with dark blue... or according to a subtag, and the rendering of 
the surface is made as water and computed according to the polygon.

With good tools (like JOSM) where members are ordered the maintenance is 
easy : it is easy to know where the succession of ways has been broken.
But would mapnik recognize this scheme ?
--
FrViPofm