This paper presents the watermarking algorithm for copyright protection of the digital geographical map composed of vector format. Most of previously related works had a weak point that degrades the accuracy as the quality of the digital geographical map or is easily destroyed by attacks. The proposed watermarking algorithm preserves the accuracy of vertex and is robust enough against attacks, and also it performs the blind watermarking by extracting the watermark without an original map. Finally, through experiments, this paper shows the proposed method is robust enough against attacks such as the elimination and modification of vertices.