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Watermarking GEO-Spatial Data - A Review

Lilian S.C. Pun-Cheng
The Hong Kong Polytechnic University
lspun@polyu.edu.hk

Zhilin Li
The Hong Kong Polytechnic University
lszlli@polyu.edu.hk

Abstract
Data copyright and security have become very important issues in geo-information technology. Digital watermarking is one of the means to protect the copyright of the data. Current researches are concentrated on the raster image data. That is, no research on geo-referenced spatial data in vector format has been known to the investigators. However, vector is still the most important data format for geo-information. It is therefore of great importance to investigate the methodology for watermarking geospatial data in vector format. Indeed, the objective of this paper is to review the applicability of existing watermarking techniques, which have been developed in computer science for digital images and CAD products, now to fundamental geospatial data, including digital elevation models (DEM), digital line graphs (DLG), digital raster graphs (DRG) and digital ortho-image models (DOM).

Introduction to Watermarking
Digital Watermarking is a digital signal processing for which the process will add some extra information to the source data. It is a kind of information-hiding techniques. Together with fingerprinting, it is classified as a type of robust copyright marking (Figure 1) (Petitcolas, Anderson & Kuhn, 1999). Its information is a hidden object, difficult to damage, destroy or steal. The main reasons for watermarking are to protect the data copyright, to keep information secret and to set a trap to any attack. The watermarking process can be very simple - define the watermarking signal and embed these signals to the data source by the watermarking algorithm (Figure 2).


Figure 1 Classification of information-hiding techniques


Figure 2, The basic work flow of digital watermarking

Any watermarking algorithm ultimately modifies the source data and result in human-made error. In general, watermarking still images will modify the image pixel values whereas watermarking a digital sound will introduce a noise of the sound. By the same principle, watermarking geospatial data will result in location inaccuracy.

A good watermarking algorithm should balance the three elements of robustness, imperceptibility and capacity. That is, the watermarking signal is hard to be detected and cracked by the attacker, imperceptible for human beings (in both vision and hearing) and the watermarked signal is large. However in general, increasing robustness will lead to a loss in imperceptibility. Conventional researches in watermarking have put much emphasis on analyzing the robustness of different watermarking algorithms against different attack mechanisms. This is performed mostly by correlating the watermarked data with the attacked data. Copying is suspected, that is watermarking exist if the coefficient is greater than the threshold value and vice versa. These techniques will be briefly introduced in the following section. It is noted that these are concerned with digital image (raster) data only.