Need help with WMS and Projections

[Ed McNierney]

Ken -

I'm having a hard time parsing your usage of "client", "my mapserver",
and "remote".  Could you explain the data flow a little more clearly?
You say that your "client" requests a "remote" layer and asks for it in
its desired projection, and if it can't get it, then "my mapserver"
somehow appears to reproject it and save the day <g>.  Where did your
mapserver come from?  Is it always there, acting as a WMS client to the
"remote WMS layer"?  What application is your "client"?  Feel free to
throw in a little ASCII art....

        - Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
Phone: +1 978 251-4242   Fax: +1 978 251-1396  

-----Original Message-----
From: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] On
Behalf Of Ken Sanderson
Sent: Thursday, December 02, 2004 7:30 PM
To: MAPSERVER-USERS at LISTS.UMN.EDU
Subject: [UMN_MAPSERVER-USERS] Need help with WMS and Projections

Hello,

I am trying to get my head around how remote WMS layers work with
projections and hoping that someone here can steer me in the right
direction.

Here is how I have assumed it works. My client goes to a remote WMS
layer and first asks for the layer in the projection of the client, if
it doesnt support that it looks through the SRS tags of the WMS layer
and finds the first one that it understands, by understands I mean thats
included in my EPSG file, requests the image and my mapserver reprojects
it to the projection my client is using.

Thats what I thought, but using a WMS layer has drawn this into
question, so is this true or not?

Here is what has brought up the problem.

I have two clients. One is in a projection of 4326 and another 42304
(LCC Canada). When I load a specific WMS layer it works in 4326, but
would not work in 42304. Here is the SRS info from the remote WMS layer:

                        <SRS>EPSG:102190</SRS>
   <SRS>EPSG:42102</SRS>
   <SRS>EPSG:3005</SRS>
   <SRS>EPSG:4269</SRS>
   <SRS>EPSG:4326</SRS>
   <SRS>EPSG:26708</SRS>
   <SRS>EPSG:26709</SRS>
   <SRS>EPSG:26710</SRS>
   <SRS>EPSG:26711</SRS>
   <SRS>EPSG:26712</SRS>
   <SRS>EPSG:26908</SRS>
   <SRS>EPSG:26909</SRS>
   <SRS>EPSG:26910</SRS>
   <SRS>EPSG:26911</SRS>
   <SRS>EPSG:26912</SRS>
   <SRS>EPSG:32608</SRS>
   <SRS>EPSG:32609</SRS>
   <SRS>EPSG:32610</SRS>
   <SRS>EPSG:32611</SRS>
   <SRS>EPSG:32612</SRS>

In my thinking since I have a number of these in my EPSG file,
specifically 4326, than I should have no problem loaded the WMS layer
into my client, regardless of what projection my client was using.

In order to get this to work in both of my clients I had to add a new
projection to my EPSG file. 102190 is BC Albers 42102 projection. As
102190 was the projection listed in the bounding box tag, I simply
copied the 42102 and put a new one into my EPSG file with the number
102190.
Works fine now. While this is working for me, I dont think its really a
good solution as I cant be adding new entries to my EPSG file for every
WMS layer my users may want to load.

Sorry for this being long, but I really wanted to give all the details.
How this has worked out has made me think my prior beliefs to how
projections with remote WMS layers worked are wrong. So my question is,
how do they work if they dont work the way I thought they did?

Thanks,

Ken