[OpenLayers-Users] HOWTO: GWC, layer MaxExtent - setting it up correctly to avoid misaligned tiles

[Phil Scadden]

Having battled with this on and off for quite a while, I think I finally 
have a solution.

If you are using WMS with a caching system, (eg GeoWebCache), and put a 
restriction on a layer extent, then you can end up with mess.

Symptoms, - misalignment of layers with basemap, repetition of parts of 
a layer on a map.

The cause is Openlayers issuing BBOX requests that dont align with GWC 
tiles.

The fix. If you are going to specify a maxExtent on a layer, you MUST 
also supply a tileOrigin

eg code.
layer = new OpenLayers.Layer.WMS(name, serverUrl, { layers: layerName, 
tiled:true, transparent: true },
  { tileOrigin: new 
OpenLayers.LonLat(map.maxExtent.left,map.maxExtent.bottom), 
maxExtent:myMaxExtent, });
-- 
Phil Scadden, Senior Scientist GNS Science Ltd 764 Cumberland St, 
Private Bag 1930, Dunedin, New Zealand Ph +64 3 4799663, fax +64 3 477 5232

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