| DEM data is one of the supportive achievements in the field of national infrastructure and earth science research, and plays a significant role in national economy and defenses. With the rapid development of digitization and network, the storage, transmission and replication of DEM data is very convenient, so the security and copyright issues have become increasingly salient. It has been recognized that the current security laws and technologies are inadequate for dealing with the behavior of infringement, disclosure, illegal dissemination and usage.Digital watermarking technology is a new information security technology, and can provide a feasible and practical solution for the copyright protection for DEM data. Digital watermarking is the process of embedding secret information into digital data, and later the information can be extracted or detected for a variety of purposes such as copyright prevention, ownership authentication, infringement tracking and tamper detection. Digital watermarking is being deemed essential to provide a strong technical support to the legitimate use of DEM data.In this paper, combining the feature, accuracy and application requirements of DEM data, the digital watermarking of DEM is systematically studied. The main results are as follows:(1) A practical digital watermarking model of raster DEM is proposed based on mapping mechanism. By this model, a mapping relationship is established between watermark sequence and elevation coordinates of raster DEM so as to resolve the no synchronization problem of each other, which can effectively enhance the robustness of watermarking algorithm in the data clip and the elevation translation. Simultaneously, this model can independently control the size of mapping tolerance to meet the accuracy requirement of DEM data and the robustness of watermarking algorithm.(2) Two algorithms are put forward based on the proposed digital watermarking model of raster DEM. The two algorithms are blind watermarking algorithm by quantification mechanism. And by the features of this proposed model, the two algorithms have the strong robustness in data clip, elevation translation, data deletion and random noise.(3) A digital watermarking algorithm of raster DEM is proposed considering the terrain features. This algorithm inherits the characteristics of the above two algorithms. In this paper, the slope and aspect are considered as two basic terrain factors. So in the process of watermark embedding, the slope and aspect are taken into account. And then using the theory of DEM error, a reasonable strategy of watermark embedding is developed to minimize the impact of watermark embedding on terrain analysis.(4) A practical digital watermarking model of TIN-DEM is established based on mapping elevation coordinates can be established, and the mapping relationship between watermark sequence and plane coordinates can also be established. This mapping relationship can effectively enhance the robustness of watermarking algorithm.(5) An algorithm considering topological features is put forward based on the established digital watermarking model of TIN-DEM. This algorithm embeds the watermark information into the X coordinates by quantification mechanism. So this algorithm can effectively maintain the elevation accuracy of DEM. In the process of watermark embedding, some constraint conditions are proposed in order to ensure the correctness of topology.(6) According to the importance of point, a digital watermarking algorithm of TIN-DEM is proposed. This algorithm inherits the characteristics of the algorithm considering topological features. And according to the importance of point, some constraint conditions are established in order to minimize the impact of watermark embedding on DEM accuracy. Simultaneously, this algorithm can achieve the purpose of"obtain the superior robustness by the small error cost".(7) According to the theory of probability and statistics, the impact rule of watermark embedding on DEM accuracy is analyzed combining the error distribution of DEM data and the algorithms proposed above. Meanwhile, the impact formula is derived theoretically. |
