Abstract: To improve the efficiency of making theme-maps for forest investigations and planning, this paper discusses the general methods and processes of making digital theme-maps based on the maps of the forest distribution. The authors establish the main principles of the combination of the subcompartment polygons, present the concept of potential-energy and the difference of potential-energy of attribute and the arithmetic of the combination of the subcompartment polygons.The authors have developed a group of programs in MapBasic based on the research of data and some key technologies of the auto-combination of polygons. The results indicate that not only the auto-combination of subcompartment polygons can be achieved, but also the efficiency and precision can be improved compared with the alternative method of interface between man and computer.