Application of satellite remote sensing to coastal topography generation ¡V A case study in Northern Taiwan
Chin-Yi Yang
Abstract
The aim of this study was to construct the coastal topography of Taiwan island by using image processing techniques with optical data.Our investigation had three main components.The first was to extract the entire waterline of Taiwanisland, the second was to generate onland information from ASTER stereo-imaging, and the last was to construct the regional tidal flat digital elevation model (DEM).
On the part of onland DEM, the ASTER onboard the EOS_AM1 platform acquires along-track stereo data for topographic mapping.The DEM was generated at 30m resolution with the highest possible level of detail.
The definition of the waterline must consider along-shore variation.In general, NIR¡]near infrared light¡^ is commonly used to delineate the waterline. Because its high absorption in the water, the reflectance from the exposed terrain to sea area declines rapidly. Due to this property, the difference in the NIR reflectance can help us to determine the position of waterline.By using image-processing techniques, the waterline of all Taiwan island were extracted.The DEM of the inter-tidal areas near the Xianshan Wetland could was also constructed from the ¡§waterline method¡¨ (Cracknell et al. 1987, Koopmans and Wang, 1995, Mason et al. 1995, Ramsey 1995).The concept of waterline method is to find the georegistered positions of the boundary between a water body and an exposed land mass from a remotely sensed image using image processing techniques, then superimposing the heights of the waterline relative to tidal level on the corresponding positions.From multiple images obtained under different tidal conditions, it was possible to build up a set of heighted waterlines within the inter-tidal zone. Gridded DEM was then generated by interpotation.
From this method, we constructed two tidal flat DEMs during 1994~1997 and 2004~2006. Comparing the changes of these two DEMs we found that¡Goutside of the Xianshan Wetland and Gangnan coastal scenic area were under erosion, other regions like south and west of Keya rivermouth, west of Haishan fishing port were under deposition. The reduction of sediment transport from the Fongshan river and the Toucian river, along with strong winter monsoon from North to South direction and the Hsinchu harbor construction could be the main reasons for Xianshan Wetland changes in the past ten years.
Reference
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