LIU Yong-Xue, ZHANG Ren-Shun, LI Man-Chun
The coast of Jiangsu Province is famous for its radial sand ridges offshore the coast, which is the largest tidal sandbanks in Chinese continent shelf. The sandbanks contain 10 great linear submarine sand ridges with center in Jiang-gang harbor. With population explosion and exacerbation of the environment, dynamic change of those sandbanks is highlighted during the past twenty years. But broad, muddy surface and complicated dynamic condition of the sandbanks, which is densely covered by tidal creeks, hinder updating of local landform data through routine measure method. The paper studies dynamic changes of the radial sand ridges offshore the coast of the Jiangsu Province based on GIS and RS technology. In the paper, a method named "superposition analysis" is brought forward and applied to the groupings of radial tidal ridges offshore the Coast of Jiangsu Province, using 9 MSS, TM satellite images (nice photographed in the low-tide period, without cloud in each image) from 1973 to 2000 and sea chart in 1979 (No. 1037), 1:50000 relief maps of research area. DEM of the radial tidal sandbanks area is created from the sea chart. Contour lines of different depth are interpolated from DEM by GIS. Pretreatment to the satellite imageries including research-area selection, atmospheric correction and image mosaic are made to the satellite images.Two crucial problems are resolved in studying dynamic change of radial tidal sandbanks: one is geometric correction of satellite images; the other is feasibility of shape comparison among satellite images acquired in different time. Key point to improve the precision of the result in studying dynamic change of the radial tidal sandbanks by satellite images of different time rests with geometric correction. Because the sandbanks are covered by sea water periodically, the shape of the radial tidal sandbanks dynamically changes with the tidal level. So we make use of the geometrical continuity of satellite images and register the land area first, which locates west of the radial tidal sandbanks. Thus in the symmetrical area extending to the sea, the error will not go far from the error in the land area. In the paper, geometric correction is made to each satellite image according to the 1:50000 relief map, with a high accuracy no more than one pixel. Based on the comparison between images on the same tidal levels, this paper makes use of series of images, that is, adopts the chart overlay analysis method, thus study the dynamic changes of radial sandbanks while avoiding the difficulty of tidal level rectification. This may be more accurate than overlaying satellite images of different tidal levels. By comparing the area of Dongsha sandbank in satellite image of 1979 with the tidal level of 114cm and the area enclosed by the 114cm contour in the sea chart, we can find that the difference of the two areas is lesser than 1% of the total area of Dongsha sandbank. Besides, the clsed shoreline enclosed by the 114cm contour of the sea chart tally with the outline of Dongsha sandbank in the satellite image of 1980 with the tidal level of 113cm on the whole. This indicates that satellite image series—the chart overlay analysis method is feasible in the application of large radial sandbanks.Appling the method mention above to the 9 phases satellite imageries, we discover that Dongsha Sandbank's aera decrease gradually. The sandbank doesn't have a simple moving trend, the motion of sandbank is rather complicated; the figure of the sandbank in the north has a southward movement; the outline of the sandbank in the west is eroded by the Xiyang Tidal Channnel; along with expanding of the Yaomen Tidal Channel in the south, the Jiangjiawu Sandbank has a trend to move to the Tiaozini Sandbank from the Dongsha Sandbank step by step. As a whole, Dongsha Sandbak lies in the range during the past twenty years.
