显示叠加的组合特征符号

怎样实现在打开图层时显示叠加的组合特征符号,比喻说交通灯等.
  //---------------------------------------
  layer := IMoMapLayer(createoleobject('MapObjects2.MapLayer'));
  layer := IMoMapLayer(map1.Layers.Item('light'));
  sym := IMoSymbol(layer.Symbol);
  ft := TFont.Create;
  ft.Name := 'ERSI US MUTCD 3';
  oleFt := FontToOleFont(ft);
  sym.Style := moTrueTypeMarker;
  sym.SymbolType := moPointSymbol;
  sym.Size := 18;
  sym.Style := moTrueTypeMarker;
  sym.CharacterIndex := 232;
  sym.Color := moBlack ;
 
  sym.CharacterIndex := 235;
  sym.Color := moGreen ;

  sym.CharacterIndex := 236;
  sym.Color := moyellow ;

  sym.CharacterIndex := 237;
  sym.Color := mored ;
  IMoLayers(Map1.layers).Add(Layer);
  //---------------------------------------