[UMN_MAPSERVER-USERS] 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.