[OpenLayers-Users] Using British OS National Grid-based WMS in openlayers... how?

[Nick Whitelegg]

Hello,

I'm developing a mashup application which combines scanned historical maps 
from npemap.org.uk with data from OpenStreetMap. It's projected in British 
National Grid (Airy projection I believe). I have developed a WMS server 
which accepts a bounding box of British National Grid eastings and northings, 
eg

http://nick.dev.openstreetmap.org/openpaths/freemaptest.php?bbox=
489000,128000,490000,129000&width=250&height=250

Valid easting ranges are approximately 0 to 600000 for eastings and 0 to 
500000 for northings.

I understand that OpenLayers will take any units for bounding box, not just 
latitude and longitude, so I should be able to use OpenLayers to do a slippy 
map for this. However I don't know what value to use for the zoom. I've tried 
a few values from 13 down to 0 but can't get a sensible bounding box centred 
on the area I want to make as the default (easting 489600, northing 128500).

Any ideas how I should tackle this? Code experiment below....

Thanks,
Nick


var map;
var popup = null;
var mode = 0;


function init()
{
    map = new OpenLayers.Map('map');


    var npe = new OpenLayers.Layer.WMS( "npemap",
        "http://nick.dev.openstreetmap.org/openpaths/freemaptest.php",
        { 'layers' : 'areas,srtm,osm', 'datasrc' : 'osm' } );
    map.addLayer(npe);
    map.setCenter(new OpenLayers.LonLat(489600,128500), 3);

}