function GetCentersQueryString()
{
	var querystring;
	var combo = document.getElementById("tipo_centro");
    var tc = combo.options[combo.selectedIndex].value;
    combo = document.getElementById("localidad");
    var cl = combo.options[combo.selectedIndex].value;
    querystring = "?tc="+tc+"&cl=".replace("amp;","")+cl;
	return querystring;
}