Because i want to estimate the water volume in lakes and reservoirs,and some of them located in different images.But the data volume is so big, so i have to think about subset them into smaller pieces and then mosaic. I’ve tried subset tool and Land/Sea Mask tool which was mentioned by lveci at this page How do creat subset image by vector polygon using S1-toolbox?. A rectangle area was used by the subset tool. And an ESRI shapefile was used by the Mask tool. The results showed that the subset tool can smaller my data volume,but the Land/Sea Mask tool was even more bigger ,like 6GB one slice! How could that happen? Can i subset my data with shapefile rather than rectangle area? I really need to smaller my dataset! Really thank you very much!