Why does this Mapserver layer not overlay correctly onto Google Maps?

Question

I'm using mapserver to serve an overlay of weather data onto a google map (V2) and the weather data is not displaying where it should.

The image below shows the problem i am having. The black lines are the land border of Alaska, as you can see, they do not line up. The brown lines represent lat long lines.

As you can see the relationship between the lat lon lines and the borders of Alaska align correctly, however they don't match up with google.

The WMS request i am using looks like the following:

http://mysite.com/cgi-bin/mapserv?map=/opt/fgs/www/htdocs/mymap.map&REQUEST=GetMap&SERVICE=WMS&VERSION=1.1.1&LAYERS=radarALASKA_TEST,Graticule&STYLES=&FORMAT=image/png&BGCOLOR=0xFFFFFF&TRANSPARENT=TRUE&SRS=EPSG:4326&BBOX=0,66.51326044311185,45,79.17133464081945&WIDTH=256&HEIGHT=256&reaspect=true

The bounding box is built using the following javascript:

var lULP = new GPoint(a.x * 256, (a.y + 1) * 256);
var lLRP = new GPoint((a.x + 1) * 256, a.y * 256);

var lUL = G_NORMAL_MAP.getProjection().fromPixelToLatLng(lULP, b, false);
var lLR = G_NORMAL_MAP.getProjection().fromPixelToLatLng(lLRP, b, false);

var lBbox = lUL.x + "," + lUL.y + "," + lLR.x + "," + lLR.y;

and this is my mapfile

MAP
#CONFIG "PROJ_DEBUG" "ON"
#CONFIG "CPL_DEBUG" "ON"
#CONFIG "MS_ERRORFILE" "/opt/fgs/www/htdocs/airlog/logs/ms_error.txt"
#DEBUG 5
IMAGETYPE PNG

SIZE 400 300
SHAPEPATH "/opt/fgs/www/htdocs/data"
#FONTSET   "/opt/fgs/www/htdocs//data/fonts.txt"
IMAGECOLOR 255 255 255 
TRANSPARENT OFF

WEB
METADATA
"wms_title"           "WMS Demo Server"
"wms_onlineresource"  "http://10.142.62.153/cgi-bin/mapserv?map=/opt/fgs/www/htdocs/mymap.map&"
"wms_srs"             "epsg:4326 epsg:900913"
END
END

PROJECTION
"proj=merc"
"a=6378137"
"b=6378137"
"lat_ts=0.0"
"lon_0=0.0"
"x_0=0.0"
"y_0=0"
"k=1.0"
"units=m"
"nadgrids=@null"
"wktext"
"no_defs"
END

EXTENT -20037508.34 -20037508.34 20037508.34 20037508.34
UNITS meters

LAYER
TYPE RASTER
STATUS ON
#NAME GISIMAGE_RADAR_CUR_ALASKA_TEST
GROUP radarALASKA_TEST
DATA "/opt/fgs/www/htdocs/Bristow/GISIMAGE_RADAR_CUR_ALASK.png"

PROJECTION
"proj=latlong"
"a=6367451.5"
"b=6367451.5"
END 
UNITS DD
PROCESSING "DITHER=YES"

# For Mapserver 3.x, we would use
# OFFSITE 0
OFFSITE 0 0 0
END

SYMBOL
NAME "circle"
TYPE ellipse
FILLED true
POINTS
1 1
END
END

#  Graticule (lines of constant lat-lon)
LAYER
NAME "Graticule"

PROJECTION
"proj=latlong"
"a=6378137"
"b=6378137"
#"ellps=WGS84"
#"datum=WGS84"
END

TYPE LINE
STATUS ON
CLASS
COLOR 180 180 180

LABEL
TYPE BITMAP
SIZE SMALL
COLOR 90 90 90
POSITION AUTO
OUTLINECOLOR 255 255 255
END
END

GRID
#  In lat-lon coordinate system, lines will always be straight (need 2 pts)
MINSUBDIVIDE 2
MAXSUBDIVIDE 180 
# MAXINTERVAL must be < MININTERVAL or mapserver will hang.
# MININTERVAL 15
# MAXINTERVAL 10
# MINARCS 18
# MAXARCS 10
LABELFORMAT '%.f'
END
END
END

I think the issue is related to the output WKT in the map file but i'm not really sure and i would certainly appreciate help from those far more experienced than me at this stuff!

Thanks everyone in advance, and if i have neglected to include some critical information, please let me know :)

Answer 1

The problem turned out to be the definition of the bounding box in the request. I needed to use the proj4.js libraries to properly define the bounding box in 3857 to get this working. Once i did this it worked perfectly, so there was nothing wrong with my mapfile after all :)

Answer 2

I have noticed that in the definition of EPSG:900913 (= EPSG:3857) on spatialreference.org differs from other definitions: The parameter "nadgrids=@null" is replaced through "towgs84=0,0,0,0,0,0,0".

I don't know the exact meaning of those parameters, so I can tell which definition in right but it might worth a try to adapt the projection in your mapfile.

(I would be interested if someone can explain the difference...)

Answer 3

You're right, projection must be transformed:

    function get_wms_url(bounds) {
         // recalculate bounds from Google to WGS
        var proj = new OpenLayers.Projection("EPSG:4326");
        bounds = bounds.transform(map.baseLayer.projection, proj);

        //construct WMS request

      var url = this.url;
        url += "&REQUEST=GetMap";
        url += "&SERVICE=WMS";
        url += "&VERSION=1.1.1";
        url += "&LAYERS=" + this.layers;
        url += "&FORMAT=" + this.format;
        url += "&TRANSPARENT=TRUE";
        url += "&SRS=" + "EPSG:4326";
        url += "&BBOX=" + bounds.toBBOX();
        url += "&WIDTH=" + this.tileSize.w;
        url += "&HEIGHT=" + this.tileSize.h;
        return url;
     }
    var layerWMS = new OpenLayers.Layer.WMS( "Roads",
        "http://"+location.host+"/cgi-bin/mapserv?map="+basePath+"&", {
            layers: 'roads,buildings,buildingssm,creek,fences,waterbodies',
            format: "image/png",
            getURL: get_wms_url,
            isBaseLayer: false
            }, {gutter: 15, isBaseLayer: false, singleTile: true} );

Answer 4

We recently installed MapServer and I also tried to use 900913. That failed. But using 3857 (the modern version of 900913) worked. So, it looks like when MapServer is installed, it may already have the 3857 definition. And you should include that in the wms_srs line of your mapfile. BTW, you might get more debug/error info if you paste the entire get map request to MapServer in a browser window and hit return. If it works with 3857, you get the png overlay. If not, hoperfully MapServer will return a line with error information to help you troubleshoot. If you still get errors, there's a projection definition area that MapServer refers to (MapServer docs says /usr/local/share/proj/epsg) that you may have to modify accordingly.