## 4 Results  
Based on the images from 21 March 2014 (Figure 6a), the trained models were run to test the dataset. The elevation estimations obtained for the testing and training datasets based on the HJ-1 image (21 March 2014) were compared to the DEM data by the waterline method. Figure 6b shows the simulated topography map for 21 March 2014. After geometric correction and histogram matching, the input parameters were extracted from the two HJ-1 images (11 July 2013 and 11 February 2012) (Figure 6c,e), input into the ANN model, and used to output the estimated elevation. These types of estimates are used to interpolate a contour map or generate a digital elevation model. Two fresh DEMs (11 July 2013; 11 February 2012) were obtained and are shown in Figure 6d,f. The resulting maps were provided with 30 m precision to facilitate this comparison according to the spatial resolution of HJ-1 image. As shown in Figure 6, the resulting DEMs are consistent with the instantaneous situation of the remote sensing images. There are some similarities and differences. On the one hand, the similarities are the trend of macro-morphology. In the Tiaozini tidal flats, the west near the land are higher than those in the east near the sea. Additionally, the elevation of the tidal flats is higher than the tidal creeks. On the other hand, the three DEMs are obviously different due to the variable tidal creeks. Tiaozini is mainly affected by the comprehensive influence of the three major tidal creeks of Xidagang, Dongdagang and Tiaoyugang (Figure 1). Due to the different hydrodynamic strength between the north and the south, the tidal creeks of Xidagang and Dongdagang oscillates very frequently, resulting in topography changes are very rapid. That is why the topography of the three DEMs looks different. The remote sensing image provides planar information that can directly display the two-dimensional shape of the Tiaozini tidal flats. Through the model of this study, the elevation information is added and help to convert from two-dimensional image to three-dimensional topography. As shown in Figure 7, 700 points evenly distributed in the Tiaozini area were selected and separated into two parts as a training (600) and testing (100) dataset. The trained models were run to test the dataset. The elevation estimations obtained for the testing and training datasets based on the HJ-1 image (21 March 2014) were compared to the initial DEM (Figure 2d) obtained by the waterline method. As shown in Figure 7a,b, the training dataset and testing dataset comparison map showed good correlation. The R\({}^{2}\) of the training dataset was 0.90, and the R\({}^{2}\) of the testing dataset was 0.91. The root mean square error (RSME) of the training dataset was 0.33 m, and the RSME of the testing dataset was 0.31 m. The error value of the training data was 21% within \(\pm\) 0.1 m, 51% within \(\pm\) 0.3 m, and 71% within \(\pm\) 0.5 m. The error value of the test data was 18% within \(\pm\) 0.1 m, 55% within \(\pm\) 0.3 m, and 76% within \(\pm\) 0.5 m.  
As shown in Figure 8, the three measured sections in 2012 and 2013 shown in Figure 8b,c were used to compare the simulated DEMs. The agreement between the cross-sections and the measured data was good, especially in the case where all three cross-sections pass through two big tidal creeks (Xidagang, Dongdagang in Figure 1). The 15 km long comparison curve of the 2013-A section (Figure 8a) crossed the Xidagang tidal creek. The accuracy was calculated by measuring points every 30 m; the RSME was 0.32 m, and the R\({}^{2}\) was 0.90. The accuracy of the 13 km long 2013-B section (Figure 8b) crossing the Dongdagang tidal creek was calculated by measuring points every 30 m; the RSME was 0.26 m, and the R\({}^{2}\) was 0.91. The 11 km long 2012-A section (Figure 8c) was distributed on the tidal flats between Xidagang and Dongdagang. The RSME of 2012-A was 0.39 m, and the R\({}^{2}\) was 0.85. Based on the comparison curve of the sections in Figure 8, the error is larger near the creeks and smaller on the flats area.