[OpenLayers-Users] Multiple labels on the same polygon

[Lehtonen, Mika]

Ok,

thanks! That makes perfectly sense, except the scale. I have that 
DOTS_PER_INCH setting set exactly as it is in the OL preview. Still, if 
I zoom to same scale, the map is approx. twice as big in my application 
than in OL preview. Does it depend on the projection? OL preview uses 
EPSG:2393 as my app. uses EPSG:900913. I really don't know if this scale 
issue makes any difference, I was just a bit curious about it.

- 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 -
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>>     
>
>
>