Cascading from the same server

[Arnulf Christl]

Howard Mark wrote:

>I'm attempting to create a single layer from a group of layers by cascading a WMS call from a server to itself.
>The WMS layer is defined in one map file and the layers it references are in another.
>An HTTP call to draw the second map (group of layers) works fine, but a call to the first map - using the WMS layer results in a "cannot draw layer" error.
>Is this cascading not possible? Or should I look for typos...?
>
>TIA for any info
>
>Mark
>
>
Hi,
yes it can be done. You can define one MAP file as WMS and use it as a
datasource in another MAP file. It doesn't make a difference if it sits
on the same server (at least it works with us).

Some things should be noticed though:
If you need projections - cascaded layers are retrieved by MapServer in
the SRS defined in the layer section
LAYER
...
   PROJECTION
      "init=epsg:4326"
   END
...
and reprojected only afterwards to match the requested EPSG resulting in
poor quality (there is only a limited number of pixels for recalculating
the reprojected image). Lables an symbology gets warped because their
positions and sizes are not recalculated but just their pixels in an
images are stretched and rotated.
Performance gets worse for every cascaded WMS, as they are requested
sequentially. If one WMS does not answer the whole application / MAP
file is blocked.

Because of these reasons we tend to rather overlay WMS in a client.

Cheers, Arnulf.

>
>
>
>********************************************************************************
>This message is intended only for the use of the Addressee and may
>contain information that is PRIVILEGED and CONFIDENTIAL.
>
>If you are not the intended recipient, you are hereby notified that any
>dissemination of this communication is strictly prohibited. If you have
>received this communication in error, please erase all copies of the
>message and its attachments and notify Space Imaging immediately.
>********************************************************************************
>
>