[OpenLayers-Users] Multiple labels on the same polygon

[Lehtonen, Mika]

Hi again Tim,

obviously I have to move on to server side caching, but I decided to 
first try TILED: true parameter. The problem with that is that not all 
the tiles are rendered in the screen. And also the multiple label on 
single polygon problem persists with certain zoom levels. With Single 
Tile everything is working fine though I probably suffer of some 
performance lack, I guess.

- mika -

Tim Schaub kirjoitti:
> Hey-
>
> Lehtonen, Mika wrote:
>   
>> Hi,
>>
>> I am trying to do some labels on polygons which are in an overlay layer 
>> on top of Google Map base layer. The polygon data is served through WMS 
>> (Geoserver). Labels are produced in GeoServer using its own style 
>> definition (TextSymbolizer). The problem is that larger polygons get 
>> more than one label on them. I would prefer them to have exactly one per 
>> polygon. If I test the polygon layer with Geoserver's OL preview, 
>> everything is just like I would want it to be. But my html page is 
>> something else.
>>
>>     
>
> Regarding labels, your WMS layer probably uses multiple tiles (unless 
> you set singleTile to true).  With multiple tile requests, the server 
> treats each as independent, so it will faithfully label as many polygons 
> as the tile bounds intersect.
>
> A good solution for nicely labeled tiles that improves performance is to 
> use something that creates meta-tiles (big tiles that are sliced into 
> smaller ones) and caches your rendered tiles.  Both TileCache and 
> GeoWebCache will do this for you.
>
> If you have data that changes frequently - or some other reason not to 
> cache tiles on the server - you can use the non-standard TILED and 
> TILESORIGIN parameters in your WMS request.  This tells GeoServer that 
> multiple requests are coming in for tiles, and it figures out which 
> polygons to label.
>
> See these two examples:
>   http://tinyurl.com/tiledfalse (TILED=false)
>   http://tinyurl.com/tiledtrue (TILED=true)
>
> Read the bit on "On the fly meta tiler":
> http://geoserver.org/display/GEOSDOC/WMS+vendor+parameters#WMSvendorparameters-TILED
>
> Also, view the source of your GeoServer layer preview to see the use in 
> an OpenLayers page.
>
>   
>> I also noticed that the scale in my page when compared to scale in the 
>> OL preview, is different. However, the SLD seem to be obeying the scale 
>> value although the real map scale is different. Could that differ be the 
>> key to my problems?
>>     
>
> OpenLayers assumes a display resolution of 72 dots per inch by default. 
>   This is almost always wrong (but an app designer has no way of knowing).
>
> The SLD spec says the following:
>
> The "standardized rendering pixel size" is defined to be 0.28mm × 0.28mm 
> (millimeters).
>
> Which (as it admits) is frequently wrong (and generally unknown by the 
> server).  But, at least it is a standard.
>
> The GeoServer layer preview sets the OpenLayers.DOTS_PER_INCH constant 
> to match the SLD "standardized rendering pixel size."  You can do this 
> as well in your application:
>
> OpenLayers.DOTS_PER_INCH = 25.4 / 0.28;
>
>   
>> I will deliver further information if needed. OL is either 2.7 or the 
>> one delivered with Geoserver 1.7.0.
>>
>>     
>
> None of the above will be different with any 2.x version of OpenLayers. 
>   But it looks like OpenLayers 2.7 got in to GeoServer 1.7 
> (http://jira.codehaus.org/browse/GEOS-2279).
>
> Good luck,
> Tim
>
>   
>> - mika -
>> _______________________________________________
>> Users mailing list
>> Users at openlayers.org
>> http://openlayers.org/mailman/listinfo/users
>>     
>
>
>