Cloud masking

Sentinel-2 cloud and cloud shaodow masking

The exclusion of clouds and cloud shadow is an important processing step which is usually done in an early preprocessing stage. The Sentinel-2 products are annotated with Quality Indicators (QI_DATA). In the Level 2A product you may find a file in the folder QI_DATA with the extension _CLD_20m.jp2 (data type: 8bit unsigned integer, spatial resolution 20m) which might be used for masking clouds. The data range is from 0 for high confidence clear sky to 100 for high confidence cloudy (for more details see L2A Product Definition Document).

1. Load a multiband Sentinel-2 satellite image (Level 2-A product) Changing Raster Layer Style to display a true color and a false color composite.
2. Zoom in to a cloudy part of the image and compare the extent of cloud and cloud shadow in both composites. Which composite is better for displaying clouds?

1. The original single band cloud quality indicator band is included in this specifc multiband file as band No. 13. Follow Split stack to extract this band.
2. Overlay the QI Cloud band on top of the color composites. Use the Identify Feature Tool for requesting pixel values of the QI band file inside a cloud. Which values represent clouds?
3. In (Layer Properties Style load Load the full data range (minimum/maximum values) ). Then change the Style to a Pseudocolor map with interpolation discrete and 10 equal interval classes as shown in the screenshot.
4. Double click on a color field of some classes (Layer Properties Style) to open the change color window. Change the opacity from 100% to 0%. Find the best threshold for clouds.

1. Create a binary cloud mask (bitmask) where clouds = 1 and non-clouds = 0. Use the Processing tool OTB Band math. You may write a conditional ifelse expression using lazy operators. Type into the expression field

im1b1>30?1:0 That means if the pixel value of band 1 of multiband image 1 is larger than 30 then replace with 1 otherwise replace with 0.

1. Apply a circular majority filter with a radius of 7 pixels.