[Tilecache] SRS parameter

[Mauclerc Anthony]

That solution could fit my needs. I am going to test it asap. Thank you. 
If I understand, the unique base layer (from a user point of view) actually corresponds to N layers (one for each projection). When the client asks for a tile, only one layer will match (the others have "wrong" projection for that request ; hopefully, those layers won't return a broken pink tile...). 
My last fear is a performance issue : if we request N layers, does it impact response time (or are layers with wrong projection efficiently ignored) ? 

Anyway, thank you for your help. I will give you back results of my experiments. 

-----Message d'origine-----
De : tilecache-bounces at openlayers.org [mailto:tilecache-bounces at openlayers.org] De la part de Christopher Schmidt
Envoyé : vendredi 21 mars 2008 19:06
À : tilecache at openlayers.org
Objet : Re: [Tilecache] SRS parameter

On Fri, Mar 21, 2008 at 05:49:33PM +0100, Mauclerc Anthony wrote:
> I'm in charge of a web application (InfoTerre) that shares french 
> geological information. In the current version, we use MapBuilder 1.0 
> on client side to request our WMS servers, with some developments 
> around that tool. One of those customizations consists in switching 
> the SRS at runtime with respect to the location and the scale : for 
> instance, the SRS is Lambert II if you are in France but switches to 
> WGS84 if you zoom out enough (if I am not clear, I let you see it at 
> http://infoterre.brgm.fr <http://infoterre.brgm.fr/> ). Of course, the 
> WMS server has to be able to deal with both projections.

Understandably. Is the place that you change it a fixed resolution/zoom level/scale?

> Presently, for performance issues, we are working on a new version 
> that embeds MapBuilder 1.5 (based on OpenLayers) and that uses some 
> raster layers as WMS-C (we use TileCache). So far, our tests are 
> promising since we can get the default raster layer on France in Lambert II.
> However, TileCache does not take SRS into account which is blocking 
> for us.

That is correct. TileCache chooses to work baesd entirely on the parameters that are set in the configuration. (This applies not only to SRS< but also to extents, resolutions, etc.: there's no way to automatically change a grid in EPSG:4326 to some other projection within
TileCache.)

> We are thinking of implementing SRS switching in TileCache (we could 
> generate independently, let's say, 3 levels in EPSG4326 and 7 levels 
> in EPSG27582, with potentialy level overlaps and requests the same 
> TileCache and the same layer in EPSG4326 and EPSG27582). I do not know 
> much about TileCache code architecture but I imagine we could add a 
> top level folder in the tile storage that specifies the SRS :

I don't think this is a good idea. Instead, I would recommend that you have your TileCache code include *two* layers: one in each projection, with options that you configure. (bbox, resolutions, etc.) Then, in your client, you configure two different layers: one in each of the underlying resolutions. Then, using something like:

http://trac.openlayers.org/ticket/1249

To reproject the point when the base layer changes from one to the other. 

This fits better within the design of OpenLayers and TileCache.  

Do you think this might fit your needs?

Regards,
--
Christopher Schmidt
MetaCarta
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