[OpenLayers-Users] Multiple labels on the same polygon

[Tim Schaub]

Hey-

Lehtonen, Mika wrote:
> 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.
> 

Right, if you really want absolute control over label position, you 
label points (or create label points for your polygons).

If there were some option to only label polygon centroid, then you would 
loose any label conflict detection/resolution.

In general, the more control you want over the behavior, the more you 
are going to have to maintain - and modify when you change your mind.

This is getting pretty far from the functionality provided by OpenLayers.

Tim

> - 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
>>>     
>>
>>
>>   


-- 
Tim Schaub
OpenGeo - http://opengeo.org
Expert service straight from the developers.