Openlayers + Mapnik + Tilecache configuration problem

Question

I am trying to setup Mapnik + tilecache but can't see any tiles in the browser when I set bbox parameters in both Tilecache.cfg and Openlayers but when I don't specify the bbox everything works fine and I can see actual map tiles.

I was wondering if anyone can point out the problem in the code. I think I have tried everything ( in my limited capability) and not really understanding why would it not work. By the way all map layers ( for mapnik styling) are sourced from a PostGIS database and have different projections and transformed on the fly by Mapnik.

OpenLayers code:

     var map, layer;

     function init(){
        var map, layer;

            var options =     {
                    numZoomLevels:20,
                    maxResolution: 360/512,
                    projection: "EPSG:4326",
                    maxExtent: new OpenLayers.Bounds(-2.0,50.0,2.0,54.0)
                    //not working when uncommented
                };

        map = new OpenLayers.Map( 'map', options);
        layer = new OpenLayers.Layer.WMS( "Map24","tilecache.py?",
                                    {
                                        layers:'mapnik24',
                                        format: 'image/png',
                                        srs: 'EPSG:4326'
                                    } );
        map.addLayer(layer);
        map.addControl( new OpenLayers.Control.PanZoomBar());
        map.addControl( new OpenLayers.Control.MousePosition());
        map.addControl( new OpenLayers.Control.LayerSwitcher());
        map.addControl( new OpenLayers.Control.Permalink("permalink"));

        if (!map.getCenter()) map.zoomToMaxExtent();
   }

Tilecache.cfg:

[mapnik24]

type=Mapnik

mapfile=/somedit/map24.xml

bbox=-2.0,50.0,2.0,54.0

levels=20

srs=EPSG:4326

projection=+proj=latlong +datum=WGS84

-- Thanks, A

Answer 1

Looking at your code I think you are asking for the region bounded by 50 and 54 degrees east, and 2 degrees north and south. Is this correct?

If it is, then I think your bounds are the wrong way around. -2 degrees (south) should be at the bottom, and 2 degrees (north) should be at the top. So the bbox should be 2.0,50.0,-2.0,54.0.

Also, looking at that region in OpenStreetMap it looks like there's not much there, is that really what you intend?

Answer 2

The OpenLayers.Bounds constructor parameters are in the order left, bottom, right top. Taking the bounds that you're using change your JavaScript to be:

            var options =     {
                numZoomLevels:20,
                maxResolution: 360/512,
                projection: "EPSG:4326",
                maxExtent: new OpenLayers.Bounds(50.0,-2.0,54.0,2.0)
                //not working when uncommented
            };

Have you tried plugging in the parameters for tilecache.py directly to see if a tile is generated?