[OpenLayers-Users] Re: WMS object

[Greg Allensworth]

On 8/8/2011 4:53 AM, Julien Cigar wrote:
>> This is a common pitfall when using OpenLayers. The SRS parameter will
>> be ignored. Always. Instead, WMS layers will be requested in the SRS
>> used by your base layer.
> Thanks for this enlightenment.. which is quite logical in fact, there is
> no reasons to have overlays in another projection than the base layer ...

Actually, we had a situation at a previous project where the remote WMS 
service was unwilling to add new a SRS to their list. Since we wanted to 
overlay their WMS onto a Spherical Mercator map, we made a simple change 
to Layer/WMS.js which adds dynamic reprojection.

I have attached the patched WMS.js file. It defines a new layer type: 
OpenLayers.Layer.ReprojectingWMS The magic is that last paragraph in 
getFullRequestString() It adds a check for layer.params.SRS and then 
reprojects the tile's bbox to make up the WMS request parameters.


Simply add the "srs" parameter to your third argument (the WMS params) 
as you would any other WMS request param.

Caveat: EPSG:23031 (a European UTM) and Google's 900913 tend to convert 
very strangely UNLESS you also load proj4js. If you're reprojecting, I 
highly recommend proj4js be used. That's easy, though: download the 
package and add the 4 <script> tags and OpenLayers will configure itself 
when it loads and detects proj4js.

Complaint: If we were to go back and clean up, we would simply assign 
OpenLayers.Layer.WMS.getFullRequestString() instead of naming it a new 
class. The WMSGetFeatureInfo control has hardcoded into it a list of 
layer types, and ReprojectingWMS is not on the list -- so this layer is 
not considered a candidate for a query. The workarounds for that aren't 
too onerous though.

-- 
Greg Allensworth, Web GIS Developer
BS  A+  Network+  Security+  Linux+  Server+
GreenInfo Network - Information and Mapping in the Public Interest
564 Market Street, Suite 510  San Francisco CA 94104
PH: 415-979-0343 x302  FX: 415-979-0371    email: gregor at greeninfo.org
Web: www.GreenInfo.org     www.MapsPortal.org

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