# Generation of Complete SAR Geometric Distortion Maps Based on DEM and Neighbor Gradient Algorithm
## 4 Discussion
### Effect of Scale Induced by the Input DEM Resolution  
It is worth noting that the above results of the SGD derived by the P-NG method are based on 30-m SRTM DEM data. Since steep and rough terrains are smoothed by lower resolution data, the use of higher-resolution DEM can of course increase the accuracy of simulated SGD by improving the identification of their obtained boundaries and depicting topographic features and complexities.  
As discussed in [PERSON] et al. [27] and [PERSON] et al. [13], the resolution of the input DEM is of great significance for the SGD simulation, and the pixel size of the input DEM should be comparable or greater with respect to that of simulated SAR data. However, their observations also confirmed that in the selection of the input DEM there was a trade-off between resolution and cost of the DEM.  
To further investigate and quantify the effect of the resolution of the DEM data on the capability of the proposed method in the mapping of the SGD, we conducted a comparison by simulating the layover and shadow with 30-m SRTM DEM as well as 90-m SRTM DEM data.  
For facilitating the comparison, the SAR intensity image (geocoded to the geographic coordinates, in Figure 9a) derived from the ALOS descending data was used as the base map. The ALOS descending data, that is, the flight pass of the satellite from the north to the south, operates in a right-looking mode, and the illumination of the test site is from the east to the west. As this image was recorded at an incidence angle of \(\sim\)38.7\({}^{\circ}\), very strong layover and shadow occurred in the intensity image, in which the layover and shadow regions can be clearly distinguished. They are both classified by visual interpretation, as depicted by the black solid lines in Figure 9, which can be viewed as the \"real layover/shadow\".  
Figure 9b,c shows the results of the layover/shadow simulation using the 90-m and 30-m SRTM DEM data, respectively. In comparison to active layover, it should be noted that the layover distortion with the passive layover is quite compatible with the \"real\" layover. However, part of the real layover is defined as foreshortening. Similarly, the imperfection can also be found from the results of the simulated shadow. Generally, the areas affected by the layover and shadow derived from 30-m DEM are wider than those from 90-m DEM. Particularly, the details of the layover and shadow identified based on 30-m DEM have been significantly increased compared with those of 90-m DEM. As expected, the higher the resolution of the input data, the more accurate the ranges of the SGD derived by the P-NG method.  
Although the performance of the P-NG method highly depends on the resolution of the used DEM, it provides us a good chance to investigate the SGD of a radar image for the study area, which can be used as an analytical reference for InSAR research in this field. The great advantage of the P-NG method is that it can be carried out at a very low cost before radar recording since the underlying DEM data such as SRTM are freely available.