[OSM-talk] Mapnik, large river, area with hole

[Thomas Walraet]

Thomas Walraet a écrit :
> Nick Black a écrit :
>>     I draw the Seine (large river) inside Paris with many areas to have
>>     holes in it.
>>
>> Why do you want to have holes in the river?
> 
> Islands...
> 
> I tried landuse areas with a layer argument, but since the rest of the 
> city don't have landuse for the moment it render the island differently. 

I add that I'm not fond of the layer approach for island. The idea of 
clockwise/anticlockwise border for water area seams far better to me.

For rendering purpose, layer approach is ok. But in reality, island are 
not in top of the water(*)... If someone wants to calculate total area 
occupied by water in a region, he will not be able to just retrieve 
water area, calculate and sum up their size.

And we can think that some areas will really be "above" water (water-ski 
area for example), so retrieving all water area that doesn't have any 
other area above it will not work either... The person who want's to 
calculate water area of a region will have to know for each possible 
area if it's considered "above water" or "not water". (which is 
difficult as anyone can add tags at will)


(*: Well... most island are not in top of the water. How to tag those 
floating cities in Andes ?)