Projection problems

[Ed McNierney]

Dave -

"Am I missing something fundamental here?"

Well, yes <g>.  First, the assumption that the Mercator projection has
"the least distortion" is only true if you are a seventeenth-century
mariner using  magnetic compass for open-water navigation!  The Mercator
projection is "familiar" because it is commonly used on world maps, but
it distorts both distance and area.  If your output map is a world map,
then it's a reasonable choice mainly because it is familiar-looking;
people expect Greenland to look larger than Brazil, even if that's not
true.

But "Transverse Mercator" is not the same as "Mercator" - they are
related but very different projections.  

Your EXTENT problem is due to the fact that your extents need to be
specified in the units of your output coordinate system/projection.  If
your output projection is in degrees, then degrees are fine.  But if you
change that you need to change the extent, too.

You cannot have a Mercator-projection map that runs from -90 to +90
latitude; it would be infinitely tall.  You cannot have a Transverse
Mercator-projection map that runs from -180 to +180 longitude; it would
be infinitely wide.

You should review your output projection decision; then use the PROJ
tools or other projection tools to calculate the new extent of your map
view, remembering that you can't show the whole planet in a Mercator or
Transverse Mercator map.

	- 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
ed at topozone.com


-----Original Message-----
From: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] On
Behalf Of Dave K
Sent: Tuesday, January 24, 2006 8:28 AM
To: MAPSERVER-USERS at LISTS.UMN.EDU
Subject: [UMN_MAPSERVER-USERS] Projection problems

Hi there,

I am having some problems reprojecting a map.  The map comes from a WMS
server and is in espg:4236 which as I understand it means that it uses
the
WGS84 datum and ellipsoid, but has no inherent projection.  I then want
to project the map using a Transverse Mercator projection (which I'm
told has the least distortion).

If I access the map without the PROJECTION clause (or with it set to
epsg:4326) the map displays fine and for some reason looks like it is in
the standard Mercator projection.

However, as soon as I include the PROJECTION clause the map becomes
completely blank.

My extents are:
  MINX=-180
  MINY=90
  MAXX=180
  MAXY=-90

... which should be displaying the entire world.

Am I missing something fundamental here?

Cheers,

Dave K

-------

MAP

  OUTPUTFORMAT
    NAME swf
    MIMETYPE "application/x-shockwave-flash"
    DRIVER "SWF"
    IMAGEMODE PC256
    FORMATOPTION "OUTPUTMODE=SINGLE"
  END

  # This bit is causing a problem
  PROJECTION
    "proj=tmerc"
    "ellps=WGS84"
    "datum=WGS84"
    "lat_0=0"
    "lon_0=0"
    "k=0.9996"
    "x_0=0"
  END

  LAYER
    NAME "demis"
    STATUS OFF
    TYPE RASTER
    CONNECTIONTYPE WMS
    CONNECTION
"http://www2.demis.nl/WMS/wms.asp?REQUEST=GetMap&LAYERS=*&FORMAT=png&TRA
NSPARENT=true&EXCEPTIONS=INIMAGE&WMS=WorldMap"

    METADATA
      "wms_title" "Demis"
      "wms_name" "*"
      "wms_server_version" "1.1.1"
      "wms_srs" "epsg:4326"
      "wms_format" "image/png"
    END

    PROJECTION
      "init=epsg:4326"

      # This is the same as:
      #"proj=latlong" (No projection)
      #"ellps=WGS84"
      #"datum=WGS84"
    END

END