function geopUrl (key, layer, format)
{  return "http://wxs.ign.fr/"+ key + "/wmts?LAYER=" + layer
      +"&EXCEPTIONS=text/xml&FORMAT="+(format?format:"image/jpeg")
      +"&SERVICE=WMTS&VERSION=1.0.0&REQUEST=GetTile&STYLE=normal"
      +"&TILEMATRIXSET=PM&TILEMATRIX={z}&TILECOL={x}&TILEROW={y}" ;
}