The demand for DEM data has dramatically increased mainly due to the large number of possible applications capable to exploit DEM data.As in many other fields

with the Internet has become preferred and effective means of data exchange

the online purchasing and distribution of DEM can now be performed relative easily.It follows that the copyright protection is increasingly becoming more and more important.Watermarking represents a potentially effective tool for the protection and verification of ownership rights in DEM.But before applying watermarking techniques developed for multimedia applications to DEM

it is important that the requirements imposed by DEM are carefully analyzed to investigate whether they are compatible with existing watermarking techniques.On the basis of these motivations

an overview of watermarking is given

and the contribution of this work is twofold:①Assessment of the requirements imposed by the characteristics of DEM on watermark-based copyright protection is compared with usual digital images.In the multimedia field

watermark must be transparent to the human visual system.While in the DEM context

this requirement can be rephrased as "not altering the elevation precision and the results of application to an extent of DEM besides transparency".This is called near-lossless requirement in this paper.It is worth noticing that near-lossless is considered more necessary than robustness requirement.That is because almost nobody would attack DEMs with degraded quality.Adaptivity is another characteristic of DEM watermark technique.Generally speaking

DEMs required high precision could be embedded in less watermark energy.While DEMs required low precision are on the contrary.Since different precision standards are established for different landforms of DEM

watermark energy embedded should also change adaptively to landforms.It ensures that the watermarked DEM meets accuracy requirements of DEM as well as watermark energy could be inserted as much as possible.In addition

efficiency of watermark algorithms should also be considered.②Discussion of a case study where the performance of two popular

state-of-the-art watermarking techniques is evaluated by the light of the requirements at the previous point.One is based on block-based DCT techniques using Watson perceptual model

the other is based on DWT techniques using PWM perceptual model.The two algorithms are applied to a 512×512 DEM of Loess Plateau with a horizontal grid spacing of 5m.The watermark is a 64×64 binary image.In the experiment

watermarked DEMs are ensured transparent to the human visual system.In order to evaluate the impact of the watermark presence on DEM

we considered histogram

slope

slope of slope

contours

collection waterlines and terrain structure lines derived from the two watermarked DEMs.It has been shown that the presence of the watermark has almost no impacts on histogram.However

the results of the applicative tasks in digital terrain analysis are heavily affected not only in the human vision but also in values.As for the DWT algorithm

from which contours

collection waterlines and terrain structure lines derived all exhibit poorer precision.Obviously

watermarking techniques evaluated by human visual system don’t always meet the requirements of elevation precision and applications of DEM.Watermark embedding should be very limited on avoiding DEM quality degradation.In the further

work will aim at the development of watermarking techniques specifically designed for DEM

so as to minimize the impact on the applications.