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An algorithm for watermark insertion and extraction on the vector image, such as map data, is presented. Most parts of the vector image, such as map data, consist of the array of the coordinate values. At first, the image is partitioned into blocks with the same area and every vertex coordinate value within a block is changed to a new coordinate value with an origin at the south-west vertex of the mask belonging to the associated block. According to the value of watermark, the insertion area is different. Unlike the work by Sakamoto et al. (Symposium on cryptography & information security, Okinama, Japan, Jan. 2000), it should be possible to use a square mask of arbitrary size. The mask of the Sakamoto et al.'s, work is specific since the length of the mask is 2R-1 where R=1, 2, ..., n. It is also shown that the proposed method is superior to the previous work against noise attack.