I was about to write a post on preprocessing Sentinel 2 data using SNAP, similar in tone to the previous one on Sentinel 1, when I realized that most of the things I was about to say are already well explained in an excellent video produced by ESA. So, we will keep the topic very short here.
In our previous post, we have already reviewed how to download data. We here show how to explore them with ESA’s SNAP tool. First, get the latest version of SNAP. In my case, I still had the same old version that I used to write the post on Sentinel 1, and that one did not work right away with Sentinel 2. So, check here and get the latest version of SNAP and the Sentinel toolbox. The data files are opened by pointing to the xml file inside the folder:
In this example we did not open the whole file, but just one of the tiles inside the “GRANULE” folder. In this case, “T33TVF” corresponding to the Vesuvius area, at the Bay of Naples, Italy. Alternatively, one might also directly access the image files inside the folder IMG_DATA, which are already rectified and projected in the corresponding UTM zones. Images are coded in 16 bit unsigned integers, with scales and offsets for all 13 bands available in the metadata for calculating radiances. The data even includes rasters for sun azimuth and elevation, which are needed for calculation of reflectance:
The images are of great quality and, at 10 m pixel size, the views are really impressive:
and even more in detail:
The MSI instrument has the unique feature of offering three bands (5, 6 and 7) specifically designed to capture the vegetation ramp, which is extremely useful for vegetation studies. Otherwise, bands 8, 4, 3 and 2 (NIR, R, G and B) closely match bands 5, 4, 3 and 2 of Landsat 8 OLI, which offers the opportunity of using Sentinel 2 MSI and Landsat 8 OLI images as complementary instruments. See, for example, this post describing a comparison of spectral responses of Landsat and Sentinel instruments.
Creating the RGB color images in SNAP is easy. Just click on the “Window” tab and choose “Open RGB Image Window”, a simple menu will open that will allow you to choose the combination you need (natural color, and IR color are preprogrammed):
Finally, one of the interesting gems in SNAP is the “Spectrum View” tool, found under the “Optical” tab, which builds spectral plots of the image at the location of the mouse pointer:
Happy working with Sentinel 2 data!