Abstract: A digital of the current study watermarking algorithm based on DCT is presented to protect the copyright of DEM data. The of the current study aim is to solve the problem that how to balance the robustness requirements of the watermark and the high precision need of the DEM. Namely the watermarking algorithm should not only have near-lossless but also robustness and be adaptive. As for the watermarking embedding strength, it is calculated by combing with the precision of the slope and aspect of the DEM and the Watson perceptual model. And by given slope and aspect precision, the watermark embedding strength can be calculated automatically. Moreover, the methodology is designed to embed a watermark in terrain lines in order to improve the robustness. The experiment results are satisfying not only in the near-lossless of the DEM precision of slope and aspect, but also in the transparency of watermark. Besides, the contour line and the SOS (slope of slope) index derived from the watermarked DEM are near-lossless as well. In addition, the watermark can resist the JPEG compression and cropping attack which means that the watermark possess robustness.