[mapserver-users] Dynamic WMS Service - Projections problem?

Tue Feb 8 09:54:49 PST 2011

First, the background...

I have a WMS Service that has an "empty" point layer in it. I use it to
display labels at dynamically generated coordinates by passing x,y within
the request. For example:

http://<domain>/ogc/mapserv?map=/path/fenceLabelWms_en.map&map.layer[fencelabel]=FEATURE
POINTS -105.5620661354989 51.14394472618872 END TEXT "Fence 1"
END&map.layer[fencelabel].class[0]=COLOR 255 0 0
END&map.layer[fencelabel].class[0]=LABEL COLOR 255 0 0 SIZE 10 POSITION UR
END&LAYERS=fencelabel&mode=map

The entire map file is as follows:

MAP
  NAME "fenceLabelWms_en"
  FONTSET "../../../mapserv/etc/fonts.txt"
  SYMBOLSET "../../../mapserv/etc/symbols.sym"
  SIZE 400 300
  IMAGETYPE PNG
  IMAGECOLOR 255 255 255
  RESOLUTION 96
  EXTENT -141.5 41.5 -51.5 83.5 # all of Canada
  UNITS METERS
  PROJECTION
    "init=epsg:4269" # lat/long
  END
  WEB
    METADATA
      "wms_srs" "EPSG:42304"
    END
  END

  OUTPUTFORMAT
    NAME png
    DRIVER 'GD/PNG'
    MIMETYPE 'image/png'
    IMAGEMODE PC256 #RGB
    FORMATOPTION "INTERLACE=OFF"
    EXTENSION 'png'
    TRANSPARENT ON
  END

  LAYER
    NAME "fencelabel"
    STATUS ON
    TYPE point
    CLASS
      SIZE 0
      COLOR 0 0 0
      LABEL
        TYPE truetype
        FONT verdana
        ANTIALIAS false
      END
    END
  END
END

A number of  applications use this WMS and each has its own projection but,
as in the request above, all pass the x,y as lat/long (EPSG:4269). The MAP
METADATA's "wms_srs" lists the supported projections (for the purposes of
this example, I'm simplifying by only including EPSG:42304). 

So, this allows me to provide the point coordinates in lat/long, but display
them using the projection from the application itself. For example:

http://<domain>/ogc/mapserv?map=/path/fenceLabelWms_en.map&map.layer[fencelabel]=FEATURE
POINTS -105.5620661354989 51.14394472618872 END TEXT 'Fence 1'
END&map.layer[fencelabel].class[0]=COLOR 255 0 0
END&map.layer[fencelabel].class[0]=LABEL COLOR 255 0 0 SIZE 10 POSITION UR
END&LAYERS=fencelabel&FORMAT=image/png&SRS=EPSG:42304&TRANSPARENT=true&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&STYLES=&EXCEPTIONS=application/vnd.ogc.se_inimage&BBOX=-2903297.000,-789911.0000,3431059.000,3959683.000&WIDTH=1219&HEIGHT=601

Knowing this, I can define this "dynamic" layer within a WMC <Layer> tag and
load it into an OpenLayers Map (whose projection is set to EPSG:42304 and
extents encompass all of Canada). The WMC definition for the above layer is
as follows (we use a batch script to set the path to the MapServer
executable and MAP file, "fenceLabelWms_en"):

<Layer queryable="0" hidden="0">
    <Server service="OGC:WMS" version="1.1.1">
        <OnlineResource xlink:type="simple"
xlink:href="http://<domain>/ogc/fenceLabelWms_en?map.layer[fencelabel]=FEATURE%20POINTS%20-105.5620661354989%2051.14394472618872%20END%20TEXT%20'Fence%201'%20END&map.layer[fencelabel].class[0]=COLOR%20255%20105%200%20END&map.layer[fencelabel].class[0]=LABEL%20COLOR%20255%20105%200%20SIZE%2010%20POSITION%20LR%20END"/>
    </Server>
    <Name>fencelabel</Name>
    <Title>Fence 1</Title>
    <SRS>EPSG:42304</SRS>
    <FormatList>
        <Format current="1">image/png</Format>
    </FormatList>
    <Extension>
        <ol:maxExtent minx="-2903297.000" miny="-789911.0000"
maxx="3431059.000" maxy="3959683.000"/>
        <ol:transparent>true</ol:transparent>
        <ol:numZoomLevels>5</ol:numZoomLevels>
        <ol:units>m</ol:units>
        <ol:isBaseLayer>false</ol:isBaseLayer>
        <ol:displayInLayerSwitcher>false</ol:displayInLayerSwitcher>
        <ol:singleTile>true</ol:singleTile>
    </Extension>
</Layer>

Okay, everything works fine up to this point. The fence label WMS is
displayed in the OpenLayers Map correctly in relation to the other layers it
contains, all of which have projections of EPSG:42304.

Now, the problem ...

I’d like to print the image displayed to the user. To do this, I have
another "empty" MAP file that I use to print the WMC; the same WMC I loaded
into OpenLayers. The entire contents of this MAP file are:

MAP
  NAME "wmsContextServer"
  FONTSET "../../../mapserv/etc/fonts.txt"
  SYMBOLSET "../../../mapserv/etc/symbols.sym"
  SIZE 400 300
  STATUS ON
  IMAGETYPE PNG
  IMAGECOLOR 255 255 255
  EXTENT -2338951 -800000 3008692 3900000
  UNITS METERS
  CONFIG "CGI_CONTEXT_URL" "1"
  PROJECTION
    "init=epsg:42304"
  END

  RESOLUTION 96

  REFERENCE
    # Parameter settings NOT to change
    STATUS          ON
    COLOR           -1 -1 -1
    OUTLINECOLOR    255 0 0
    # Parameter settings that will change if you replace the default
'refmap.png'
    IMAGE           "../htdocs/images/refmap.png"
    SIZE            100 74
    EXTENT          -2.2e+006 -712631 3.0728e+006 3.84e+006
  END # REFERENCE

  SCALEBAR
    STATUS EMBED
    INTERVALS 1
    POSITION LL
    TRANSPARENT true
    UNITS kilometers
    STYLE 1
    SIZE 100 7
    LABEL
      SIZE small
    END
  END

  WEB
    INCLUDE "../../../mapserv/conf/imagepath.inc.map"
    METADATA
      # Used for App Page Title
      "wms_title" "WMC Context Client"
      "wms_connectiontimeout" "60"
    END
  END
  OUTPUTFORMAT
    NAME png
    DRIVER 'GD/PNG'
    MIMETYPE 'image/png'
    IMAGEMODE RGB
    FORMATOPTION "INTERLACE=OFF"
    EXTENSION 'png'
    #TRANSPARENT ON
  END

  OUTPUTFORMAT
    NAME gif
    DRIVER 'GD/GIF'
    MIMETYPE 'image/gif'
    IMAGEMODE RGB
    FORMATOPTION "INTERLACE=OFF"
    EXTENSION 'gif'
  END

END

Unfortunately, the "dynamic" WMS layer doesn't appear in the output image. I
can use the following URL to get an image of all the other layers in the WMC
successfully, but the fence label is not shown and there is no error
message:

http://<domain>/ogc/mapserv?map=/path/wmsContextServer.map&mode=map&context=http://<domain>/contexts/WMCwFenceLabels.xml&layers=all

I'm guessing MapServer is getting a blank image when it's assembling the
layers for the output image and I'm further guessing that this is the result
of the projections somehow getting out of whack somewhere along the line.
I'm thinking that the point is being evaluated as not actually existing
within the extents of the output image being generated, perhaps? But, I
can't guess why this would be the case. There is no question that it does
fall within the extents (as witnessed by loading the same WMC in
OpenLayers).

As it is now, I am completely stuck as to what the problem might be. Can
anyone suggest anything that might help me narrow down exactly what the
problem is or a direction I can investigate further? 

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