[OpenLayers-Users] Multiple labels on the same polygon

[Eric Lemoine]

Hi. Do we agree that metatiling does not completely solve the issue?
I'm thinking about the "polygon spans multiple metatiles" case. Eric

2008/10/28, Tim Schaub <tschaub at opengeo.org>:
> 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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>
>
> --
> Tim Schaub
> OpenGeo - http://opengeo.org
> Expert service straight from the developers.
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