[OpenLayers-Users] Does OpenLayers provide a way to check if a layer is available?

[Phil Scadden]

is the 500 error persistent? (which would allow a change in your layer 
list).
You can detect an issue with:
             layer.events.register("tileerror",layer,function(evt){
                  *your code*
}
but quite what you do in such a case is more interesting. Possibilities 
- add both layers to map but make on invisible. On error, switch 
visibility and return a transparent tile
               evt.tile.setImgSrc("images/empty.png");
               return false;
Good luck!

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