อยากสอบถามเรื่องการตรวจสอบ file xml ครับมันผิด XML namespace is wrong แก้อย่างไรดีครับ

protected void Page_Load(object sender, EventArgs e)
        {
            if (!Page.IsPostBack)
            {
                TestTest();
            }
        }
        static DataTable Table()
        {
            clsDatabase clsDB = new clsDatabase();
            DataTable table = new DataTable("url");

            table.Columns.Add("loc", typeof(string));


            table.Rows.Add("http://www.test1.com/");
            table.Rows.Add("http://www.test1.com/new.asp");
           
            return table;
        }

        private void TestTest()
        {
            DataSet set = new DataSet();
            set.Tables.Add(Table());
            System.Xml.XmlTextWriter writer = new System.Xml.XmlTextWriter(Server.MapPath("sitemap.xml"), System.Text.Encoding.UTF8);
            writer.WriteStartDocument();

            writer.WriteStartElement("urlset", "http://www.sitemaps.org/schemas/sitemap/0.9");
            writer.WriteAttributeString("xmlns", "xsi", null, "http://www.w3.org/2001/XMLSchema-instance");
            writer.WriteAttributeString("xsi", "schemaLocation", null, "http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/09/sitemap.xsd");
            set.WriteXml(writer);
            writer.WriteEndDocument();
            writer.Close();

        }

<?xml version="1.0" encoding="UTF-8"?>
-<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/09/sitemap.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
-<NewDataSet xmlns="">
-<url>
<loc>http://www.test.com/</loc>
</url>
-<url>
<loc>http://www.test.com/new.asp</loc>
</url>
</NewDataSet>
</urlset>

            DataTable dt = new DataTable();
            dt.Columns.Add("loc", typeof(string));

            dt.Rows.Add("http://www.test1.com/");
            dt.Rows.Add("http://www.test1.com/new.asp");

            WriteSiteMapXml(string.Format("{0}\\sitemap.xml", Application.StartupPath), "url", dt);

        public void WriteSiteMapXml(string xmlPath, string tagName, DataTable dtData)
        {
            if (File.Exists(string.Format("{0}\\sitemap.xml", Application.StartupPath)))
                File.Delete(string.Format("{0}\\sitemap.xml", Application.StartupPath));

            XmlTextWriter writer = new System.Xml.XmlTextWriter(xmlPath, System.Text.Encoding.UTF8);
            writer.WriteStartDocument();
            writer.WriteStartElement("urlset", "http://www.sitemaps.org/schemas/sitemap/0.9");
            writer.WriteAttributeString("xmlns", "xsi", null, "http://www.w3.org/2001/XMLSchema-instance");
            writer.WriteAttributeString("xsi", "schemaLocation", null, "http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/09/sitemap.xsd");

            writer.Indentation = 0;
            writer.Formatting = Formatting.Indented;

            foreach (DataRow dr in dtData.Rows)
            {
                writer.WriteStartElement(tagName);

                foreach (DataColumn dc in dtData.Columns)
                {
                    writer.WriteElementString(dc.ColumnName, (dr[dc.ColumnName] != null) ? dr[dc.ColumnName].ToString() : string.Empty);
                }

                writer.WriteEndElement();
            }

            writer.WriteEndElement();
            writer.WriteEndDocument();

            if (writer != null)
            {
                writer.Flush();
            }

            if (writer.WriteState != WriteState.Closed)
            {
                writer.Close();
            }
        }