Merging GridView Header Columns Multiple Headers ASP.NET

In Aspx Page

<asp:GridView ID="GridView1" runat="server"  DataSourceID="SqlDataSource1" 

OnRowCreated="GridView1_RowCreated" ShowHeader="False">

<Columns>
<asp:BoundField DataField="DepartMentID" HeaderText="DepartMentID"/>
<asp:BoundField DataField="DepartMent" HeaderText="DepartMent"/>
<asp:BoundField DataField="Name" HeaderText="Name"/>
<asp:BoundField DataField="Location" HeaderText="Location"/>
</Columns>
</asp:GridView>

<asp:SqlDataSource ID="SqlDataSource1" runat="server" 
ConnectionString="<%$ ConnectionStrings:ConnectionString %>"
SelectCommand="SELECT [DepartMentID], [DepartMent], [Name], 
              [Location] FROM [Employee]">
</asp:SqlDataSource>

In C#

protected void GridView1_RowCreated(object sender, GridViewRowEventArgs e)
    {
        if (e.Row.RowType == DataControlRowType.Header)
        {
            GridView HeaderGrid = (GridView)sender;
            GridViewRow HeaderGridRow = new GridViewRow(0, -1, DataControlRowType.Header, DataControlRowState.Insert);
            TableCell HeaderCell = new TableCell();
            HeaderCell.Text = "Department";
            HeaderCell.ColumnSpan = 2;
            HeaderGridRow.Cells.Add(HeaderCell);

            HeaderCell = new TableCell();
            HeaderCell.Text = "Employee";
            HeaderCell.ColumnSpan = 2;
            HeaderGridRow.Cells.Add(HeaderCell);

            GridView1.Controls[0].Controls.AddAt(0, HeaderGridRow);

   HeaderGridRow = new GridViewRow(0, 0, DataControlRowType.Header, DataControlRowState.Insert);
            HeaderCell = new TableCell();
            HeaderCell.Text = "DepartMentID";
            HeaderCell.RowSpan = 1;
            HeaderGridRow.Cells.Add(HeaderCell);

            HeaderCell = new TableCell();
            HeaderCell.Text = "DepartMent";
            HeaderCell.RowSpan = 1;
            HeaderGridRow.Cells.Add(HeaderCell);

            HeaderCell = new TableCell();
            HeaderCell.Text = "Name";
            HeaderCell.RowSpan = 1;
            HeaderGridRow.Cells.Add(HeaderCell);

            HeaderCell = new TableCell();
            HeaderCell.Text = "Location ";
            HeaderCell.RowSpan = 1;
            HeaderGridRow.Cells.Add(HeaderCell);

            GridView1.Controls[0].Controls.AddAt(1, HeaderGridRow);

        }
    }