Abstract: Objectives: Agglomeration is an important operator for map generalization of building groups, which can solve the visual conflicts between building symbols and maintain the independence of individual buildings simultaneously. However, the existing methods of agglomeration are difficult to maintain the orthogonality and spatial distribution of building groups. This paper proposes an algorithm of building groups agglomeration considering the overall orthogonal features of building polygons, based on identifying the facing projection segments between building polygons automatically. Method: Firstly, all facing projection segments meeting the criteria of agglomeration can be found rapidly using the regular grid index of original data. Then, the agglomeration main direction lines are generated based on sets of facing projection segments associated with orthogonal segments of original building polygons. Finally, according to the degree of association between the vertices of the facing projection segments and the generated agglomeration main direction lines, the new vertices on the agglomeration lines are generated. And the original vertices of building polygons related with these new vertices must be adjusted. Results: The building footprints data at large-scale in a part of Guangzhou city is the experimental data. The algorithm of building groups agglomeration has been implemented for different kinds of building groups. The change of five indices describing spatial characteristics between before and after the agglomeration of building groups in this experiment has been analyzed. Conclusions: The experimental results have verified the effectiveness and applicability of the algorithm proposed in this paper. This algorithm proposed in this paper can directly generate the agglomeration lines between building polygons directly by means of paired facing projection segments, while the overall orthogonal features of building polygons agglomerated can be preserved based on agglomeration main direction lines.