Test Data and Results
Figure 3.1 is a SPOT multi-spectral image acquired on Nov. 12, 1998. It is a shelf near western coast of Taiwan. The red line is detected shoreline by proposed scheme. The yellow line is a ground survey result using DGPS(Differential Global Positioning System) at the same time with the accuracy under one meter. The error checking positions are indicated as green segments. Table 3.1 is the error of each segment.

Figure 3.1 Test SPOT images.

Table 3.1 Error of each segment. Segment No. Absolute Length(m) 1 20 2 22 3 19 4 14 5 32 6 15 7 13 8 16 9 19 10 26 11 13 12 16 13 14 14 15 15 18

From table 3.1, the error is around one image pixel. This result illustrates the practicability of proposed scheme. In figure 3.2, a series of SPOT images acquired during Nov. 1 and Nov. 12, 1998. The location of these images is the southern coast of Taichung Harbor. In this period, the sea level varies from 85.8cm to 515.6cm. Figure 3.3 shows the detected shorelines and demonstrates the contours of tideland.

11/03/1998, 515.6cm	11/02/1998, 451.3cm	11/01/1998, 339.3cm
11/12/1998, 125.8cm	11/11/1998, 85.8cm

Figure 3.2 A series of test SPOT images and their sea levels.





Figure 3.3 The detected shorelines of tideland.
Figure 3.4 is two SPOT images at a time interval of 5 years over Taichung Harbor. Their sea levels are 122.7cm and 125.8cm, respectively. Figure 3.5 demonstrates the time variation of the tideland over the same area.

12/08/1993, 122.7cm	11/12/1998, 125.8cm
Figure 3.4 Two SPOT images at a time interval of 5 years and their sea levels.

Figure 3.5 The time variation of the tideland over the same area.
Conclusion
From the comparison of detected shoreline and ground survey, the results illustrate the proposed scheme is capable of detecting shoreline automatically.

Obviously, if a series of images within a short period of time can be collected, we can delineate contours of the tideland. Accordingly, if a series of images with their time interval is long enough and the sea levels are similar, we can detect the variation of tideland. From the result of the test data shows that the tideland of the southern coast of Taichung Harbor is shrinking.

Reference

• Conrac Corp., “Raster Graphics Handbook”, Conrac Division, 1980.
• Rafael C. Gonzalez and Richard E. Woods, “Digital Image Processing”, Addsion-Wesely, 1992.