A lot of data of elevation in different places of the shallow water of Nantong were obtained by fish exploring machine and GPS determining and tide correcting.An image which reflects landforms of Nantong was obtained by selecting from the many NOAA images on different channels and in different phases.Based on the image,grey scales corresponding to different places were obtained.By using classifying liner regression technique,liner regression equations were established between the elevation and grey scale,and the threshold values of grey scales of the different elevations were determined.According to the threshold values,the areas of the tidal zone above different elevations of the shallow water of Nantong were estimated.