I need to extract land cover information (vegetation, water bodies, etc.) using Landsat 8 scenes. I have to extract the information on a country scale which means that I have about 15 scenes (different Rows and Paths and different days of the year --> 2015). I am using the Semi-Automatic Classification plugin in QGIS in order to download and pre-process the scenes (conversion from DN to TOA and DOS1 correction). What I thought I should do is pre-process the scenes separately and then merge the scenes for the different bands (i.e. in the end I will end up with one band 1 where I have merged all pre-processed bands 1 of the separate scenes, then one band 2 with all pre-processed bands 2 of the separate scenes, etc....). The image below shows an example of what happens when I merge the first 3 pre-processed scenes of band 2:
You can see clearly the boundaries between the three scenes, but what I find troubling is that when I check the raster values of the same object at the interface between two scenes, the raster values differ significantly. My questions are:
- Is it even correct to patch Landsat images in the way I am doing it (since the scenes are from different days of the year). If this is not correct is there a way in which I can accomplish what I am trying to do?
- Is it possible to end up with smoother patched up image of the different scenes?
- Why are the raster values belonging to the same object in two different scenes so significantly different? Is it due to the fact that images are taken in different days?