Strange clipping on polar stereographic maps.

[Dave Weaver]

I'm experimenting with MapServer and have come across a problem I
don't understand.

I have a basic (vector) world map, overlaid with a (raster) satellite
image. I'm accessing it using the WMS interface. When I display it in
EPSG:4326 projection it looks fine, like this:

http://server6.pictiger.com/img/39525/computer-games-and-screenshots/coverage-of-image.png

However, if I change the projection to Polar Stereographic, I get some
strange clipping issues.
Take a look at the following 4 images. They are all of the same data,
centred over the north pole. Each image is more zoomed out than the
previous one (I was just changing the BBOX parameter in the URL used
to access the images).

http://server6.pictiger.com/img/39526/computer-games-and-screenshots/bounding-box-at--2000000%2C-2000000%2C2000000%2C2000000-.png
http://server6.pictiger.com/img/39527/computer-games-and-screenshots/bounding-box-at--4000000%2C-4000000%2C4000000%2C4000000.png
http://server6.pictiger.com/img/39528/computer-games-and-screenshots/bounding-box-at--6000000%2C-6000000%2C6000000%2C6000000.png
http://server6.pictiger.com/img/39529/computer-games-and-screenshots/bounding-box-at--8000000%2C-8000000%2C8000000%2C8000000.png

Notice how, as we zoom out, parts of the word disappear. Also, the
satellite image is clipped lower and lower each time.

What's going on here?

If it's relevant, the URLs being used to access the images are like this:

http://example.com:8080/cgi-bin/mapserv?map=/home/map/fgs/www/htdocs/maps/pstest.map&SERVICE=WMS&VERSION=1.1.1&CRS=EPSG:9810&REQUEST=GetMap&LAYERS=world,sat,radar&WIDTH=800&HEIGHT=700&FORMAT=image/png&TRANSPARENT=false&BBOX=-2000000,-2000000,2000000,2000000

and the mapfile being used here is:

MAP
  NAME		"Test Map"
  STATUS	ON
  IMAGETYPE     PNG
  SHAPEPATH      "/home/map/fgs/www/htdocs/maps/data"
#  EXTENT        -180 -89 180 89
  IMAGECOLOR    100 100 200
  SIZE          800 400

  WEB
    METADATA
	"wms_title"		"Demo WMS Server"
	"wms_onlineresource"	"http://example.com:8080/cgi-bin/mapserv?map=/home/map/fgs/www/htdocs/maps/test2.map&"
    END
  END

  PROJECTION
     "proj=stere"
     "lat_0=90"
     "lon_0=0"
  END

  LAYER
    NAME         world
    METADATA
	"wms_title"	"World"
    END
    DATA         WORLD
    STATUS       ON
    TYPE         POLYGON

    PROJECTION
      "init=epsg:4326"
    END

    CLASS
      NAME       "The World"
      STYLE
        COLOR        100 232 100
        OUTLINECOLOR 32 32 0
      END
    END
  END

  LAYER
    NAME    "sat"
    METADATA
	"wms_title"	"Satellite"
    END
    DATA    "/home/map/fgs/www/htdocs/maps/images/sat1.tiff"
    STATUS  OFF
    TYPE    RASTER

   PROJECTION
     "init=epsg:4326"
   END
  END

  LAYER
    NAME    "radar"
    METADATA
	"wms_title"	"Radar"
    END
    DATA    "/home/map/fgs/www/htdocs/maps/images/radar1.tiff"
    STATUS  OFF
    TYPE    RASTER

   PROJECTION
     "init=epsg:4326"
   END
 END

END

Any feedback appreciated.
Many thanks,
Dave.