Need help with WMS and Projections

[Daniel Morissette]

(I have CC'd this reply to the list for the benefit of other users who
may have the same questions, see my answers below...)

Ken Sanderson wrote:
> Thanks Daniel for the reply. Just to make sure I understand what you
> have stated below.
>
> I want to load a remote WMS layer (one thats out of my control, served
> by another server), its SRS is as you described is:
>
> "wms_srs" "EPSG:102190 EPSG:42102 EPSG:3005 EPSG:4269 EPSG:4326
> EPSG:26708 EPSG:26709 EPSG:26710 EPSG:26711 EPSG:26712 EPSG:26908
> EPSG:26909 EPSG:26910 EPSG:26911 EPSG:26912 EPSG:32608 EPSG:32609
> EPSG:32610 EPSG:32611 EPSG:32612"
>
> Now first my chameleon client checks to see if it will support my output
> projection of 42304 and goes through the wms_srs list, which in this
> case it doesnt as its not in that list at all. So then it grabs the
> first projection listed, which is 102190, which my client's mapserver
> didnt understand as it wasnt listed in my EPSG file. It wont look to
> 42102 or any beyond that I do have, it just stops at the first one?
>

Correct. It just stops at the first one.

> Is that all correct? So if I am going to serve up a WMS layer its likely
> best if I make my first SRS something very common like EPSG:4326 instead
> of a unique like 102190?
>

Yep, in the context of MapServer you are better put the most common
projection at the beginning. I don't know how other clients are
implemented, but I wouldn't be surprised if many other WMS clients would
have used the same strategy.

We could potentially enhance MapServer to try to be smart and pick the
projection from the wms_srs list that would create the least distortion
when warping the image it gets from the remote WMS, but that could be
very complex to implement and result in lots of overhead at runtime so
I'm not even sure if that's a good idea.

Daniel
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  Daniel Morissette               dmorissette at dmsolutions.ca
  DM Solutions Group              http://www.dmsolutions.ca/
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