OpenLayers, how restricting WMS layer extent - gis

Question

I'm able to create an OpenLayers Map with maxExtent and restrictedExtent properties.
Also I have play with WMS layer and its maxExtent property, but the problem comes when use singleTile in the WMS layer.
What I want is to have a WMS layer that gets only one image from server but restricted to the bounds I want. Because this I used singleTile=true, ratio=1 and maxExtend=my_desired_extent.
But it always request the whole map view.
Any ideas how to achieve it.
Thanks in advance.
Next code shows a map with a base layer and a costline line layer I would like to use in singleTile mode and limiting the are they must request to WMS:
<script type="text/javascript" src="http://openlayers.org/api/2.11/OpenLayers.js"></script>
<div id="map" style="width: 700px; height: 500px;"></div>
<script type="text/javascript">
var map = new OpenLayers.Map("map");
var wms = new OpenLayers.Layer.WMS("OpenLayers WMS Basic", "http://labs.metacarta.com/wms/vmap0",
{
layers: 'basic'
});
map.addLayer(wms);
var restricted = new OpenLayers.Layer.WMS("Coast Line", "http://labs.metacarta.com/wms/vmap0",
{
layers: 'coastline_01,coastline_02'
},
{
opacity: 0.6,
singleTile: true,
ratio: 1,
isBaseLayer: false,
maxExtent: OpenLayers.Bounds.fromString("-10,-90,30,90")
});
map.addLayer(restricted);
map.setCenter(new OpenLayers.LonLat(0,40), 3);
</script>
I'm looking at OpenLayers source and seems what I want is not implemented so I need to create some WMS subclass or redefine some WMs layer methods.
I would appreciate any help.
Thanks.

Answer 1

If you are looking at restricting the whole map to the extent your talking about and not allowing users to pan outside of the extent you can try something like the following.
var extent = new OpenLayers.Bounds({-10,-90,30,90});
function init() {
var options = {
restrictedExtent: extent
};
map = new OpenLayers.Map('map', options);
var restricted = new OpenLayers.Layer.WMS("Coast Line", "http://labs.metacarta.com/wms/vmap0",
{
layers: 'coastline_01,coastline_02'
},
{
opacity: 0.6,
singleTile: true,
ratio: 1,
isBaseLayer: false
});
map.addLayer(restricted);
The difference here is that I have set the extent on the whole map not just the layer. Note that you will need to control the maximum zoom level as well. If you don't a user can still zoom out and data passed the extents will be shown though they will not be able to pan the view on the map.
If you want the map to pan outside of the extent specified but just not that layer then there are a number of approaches.
If you have access the the WMS server that the layer is served from you could add a setting there to restrict the extent of the layer server.
Check this link http://docs.geoserver.org/latest/en/user/webadmin/data/layers.html and read the "Bounding Boxes Section"
If you don't have access to the WMS server and you just want to restrict the area that is requested from the WMS server from the client side you could try using the OpenLayers.layer.WMS class with a different strategy. See below for an example:
var myfilter = new OpenLayers.Filter.Spatial({
type: OpenLayers.Filter.Spatial.BBOX,
value: OpenLayers.Bounds.({-10,-90,20,90})})
var filterStrategy = new OpenLayers.Strategy.Filter({filter: myFilter});
var restricted = new OpenLayers.Layer.WMS("Coast Line", "http://labs.metacarta.com/wms/vmap0",
{
layers: 'coastline_01,coastline_02'
},
{
opacity: 0.6,
singleTile: true,
ratio: 1,
isBaseLayer: false,
strategies: [filterStrategy]
});
map.addLayer(restricted);
Please note that I have not tested this code and it may take a little refining to get it to work.