[OSM-talk] Islands

Sat Apr 14 12:57:20 BST 2007

On 14 Apr 2007, at 11:16, 80n wrote:

> On 4/14/07, Artem Pavlenko <artem at mapnik.org> wrote:
>
>
> On 14 Apr 2007, at 10:47, 80n wrote:
>
>> David
>> I think there are a couple of problems with closing off the ends  
>> of the river sections.
>>
>> Firstly, purists would argue that a segment crossing the river is  
>> an artifice that does not represent anything in the real world.
>>
>> Second, you would need two segments, or the segment needs to  
>> belong to two ways (the upstream way and the downstream way).  If  
>> it belongs to two ways then for one of them it is pointing in the  
>> wrong direction. (segment sorting will fix that, but that requires  
>> running frollo before Osmarender).
>>
>> I've contemplated doing this myself several times and always  
>> managed to persuade myself not to.
>>
>> 80n
>
> 80n,
> We don't really need all this crossing segments.
> Please, could you explain to me what's wrong with representing  
> features like 'river with islands' as proper polygons (one exterior  
> and n-interior rings) ? If you want just a small part of a bigger  
> polygon for rendering use _clipping_.
>
> Artem
> That's what I said wasn't it?  There should not be segments  
> crossing the rivers because:
>
> "Firstly, purists would argue that a segment crossing the river is  
> an artifice that does not represent anything in the real world.
>
> Second, you would need two segments, or the segment needs to belong  
> to two ways (the upstream way and the downstream way).  If it  
> belongs to two ways then for one of them it is pointing in the  
> wrong direction. (segment sorting will fix that, but that requires  
> running frollo before Osmarender)."
>

Yes, you did. I was thinking about general use of polygons for  
features like  rivers, lakes etc.
See: http://www.informationfreeway.org/? 
lat=6703451.74288&lon=34320.21826&zoom=13&layers=B000

Cheers,
Artem

> 80n
>
> Cheers,
> Artem
>
>
>
>>
>> BTW I noticed an error on the Baghdad map.  The al-Sarafiya bridge  
>> is still shown, but it was blown up last week ;)
>>
>> On 4/14/07, David Groom <reviews at pacific-rim.net > wrote:
>> If the ends of the riverbank were closed off with segments so the  
>> riverbank formed a complete polygon, would the method outlined  
>> below by 80n be enough to render rivers and islands correctly in  
>> the mapnik layer?
>>
>> David
>> ----- Original Message -----
>> From: 80n
>> To: David Groom
>> Cc: talk at openstreetmap.org
>> Sent: Saturday, April 14, 2007 9:21 AM
>> Subject: Re: [OSM-talk] Islands
>>
>> One method that currently works is to tag islands as part of the  
>> same way as the riverbanks, but make them go anti-clockwise.
>>
>> So you have one way that goes up the left bank of the river for,  
>> say, 1km then goes down the right bank for for the same distance.   
>> If the two riverbanks are imagined to be part of a closed way then  
>> the segments should point in a   clockwise direction. Then all of  
>> the islands within that 1km section are also made part of that  
>> same way, but tagged in an anti-clockwise direction.
>>
>> This seems to work pretty well, even when cut at arbitrary points  
>> by a bbox, and is understood by SVG so renders with Osmarender.   
>> The same principle also workd for islands in lakes and any other  
>> situation where something has a "hole" in it.
>>
>> There are some examples along the River Thames:
>>
>> Simple example: http://www.informationfreeway.org/? 
>> lat=6698535.88756&lon=-57979.86205&zoom=16&layers=B000
>> Complex example: http://www.informationfreeway.org/? 
>> lat=6688981.2611&lon=-51110.08562&zoom=16&layers=B000
>>
>> 80n
>>
>> On 4/14/07, David Groom < reviews at pacific-rim.net> wrote:
>> I don't know :)
>>
>> I've been meaning to update the large rivers page [1] to get  
>> debate going on the whole large rivers  / lakes idea, which would  
>> include a discussion on how to render islands.
>>
>> Straying away from your initial question, and looking at large  
>> rivers, Osmarender currently renders large rivers, but the Mapnik  
>> layer does not.
>>
>> As I understand it Mapnik needs a proper polygon to generate the  
>> fill and so does not generate a filled wide river from the  
>> proposed rendering on the large rivers [1] page.  Own its own this  
>> is fairly easy to implement.
>>
>> Moving now to your question about islands :
>>
>> It is, as far as I can see, fairly difficult to intuitively split  
>> a river with islands in it into polygons (particularly where there  
>> are many islands close to each other) , where each polygon would  
>> be a continuous way and there is not a complicated system of  
>> segments going between the riverbank and each island.
>>
>> I'd like to see a modification of my proposal on the large rivers  
>> page [1] where riverbanks would be drawn as a polygon with a tag  
>> something like waterway = riverbank, and islands in these rivers  
>> woul d be drawn as waterway = riverbank; island = yes.  Given this  
>> suggested rendering, island in lakes could be tagged exactly the  
>> same as islands in rivers and they would still render correctly.
>>
>> David
>>
>>
>> [1]  http://wiki.openstreetmap.org/index.php/Proposed_features/ 
>> Large_rivers
>>
>>
>> ----- Original Message -----
>> From: lewispusey
>> To: talk at openstreetmap.org
>> Sent: Friday, April 13, 2007 5:08 PM
>> Subject: [OSM-talk] Islands
>>
>> What is the current best way to draw and tag islands in rivers and  
>> lakes?
>> Lewis
>>
>>
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>
> Artem Pavlenko
> http://mapnik.org
>
>
>
>

Artem Pavlenko
http://mapnik.org

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