Scaleless GIS is one of the key technologies of digital earth and web gis. With the development of GIS

the current GIS techniques couldn't meet the requirements of the information society very well. The ability of deriving small scalem ap from large scale map automatically is very important in modern GIS. To derive small scale map from large scale map in scaleless GIS is related to cartographic map generalization technique. Based on the description of the amount rule of selection

quality principle of selection

combination principle and attribute data generalization. A hierarchical organization strategy of polygon map layer

C Tree

and the corresponding algorithm of generalization in scaleless GIS is presented in this paper. The algorithm based on C Tree can complete the polygon selection and combination operation in high efficiency for given large scale polygon map layer and small output scale. This strategy has been implemented into a 863 project of China and the experimental results show that it can complete polygon layer generalization with high efficiency and can maintain the appearance and the topological structure of map well.