[OpenLayers-Users] WMS not shown below a certain scale

[Tim Schaub]

Hey-

Till Adams wrote:
> Dear list!
> 
> I have an overlay of MS Ve data with two WMS coming from UMN Mapserver.
> 
> Although there is no minscale argument defined in the UMN config file, 
> my open Layers client
> grays out both WMS when zooming into a certain zoomlevel and does not 
> show my WMS on the map.
> The MS VE map is still there.
> 
> I would expect that the wms is only greyed out, when a certain min or 
> maxscale is reached.
> 
> Does anybody have an idea on that?

Sounds like you are seeing the layer names grayed out in the layer 
switcher.  This has to do with the minResolution and maxResolution 
properties of the layer - on the client side.  OpenLayers knows nothing 
about your mapfile.  So, you'll want to set the minResolution and 
maxResolution properties of your wms layers to some appropriate level.

A nice way to determine the current map resolution is to get Firefox and 
install Firebug.  Open the Firebug console and type:

map.getResolution()

Or something different if you have named your map something besides 
"map."  Figure out what min/maxResolutions you do want for your layers, 
and then construct them appropriately.  The min/maxResolution properties 
belong on the options argument to the WMS layer constructor.

That is

var layer = new OpenLayers.Layer.WMS(
     "Layer Name",
     url,
     params,
     {minResolution: smallNumber, maxResolution: bigNumber}
);

The units for resolution are map units per pixel.

Note that scale is not well preserved in web mapping.  Your mapfile 
might assume a client screen resolution of 96 dpi, OpenLayers might 
assume 72 dpi, and your real monitor resolution (or those of viewers 
elsewhere in the world) is unknown.  So, stick with resolution (where 
resolution is defined by map units per pixel).  If you do have to use 
scale, ensure that your mapfile setting for DPI and your OpenLayers 
setting for DPI are the same.

Mapfile snip:

     RESOLUTION 96 # dots per inch (default is 72)

OpenLayers config:

     OpenLayers.DOTS_PER_INCH = 96;

Tim



> 
> Regards, Till
> 
> 
> 
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