Public repository for GRASS GIS module for clouds and shadows detection (GSoC 2018 project)
Optical sensors are unable to penetrate clouds leading to related anomalous reflectance values. Unlike Landsat images, Sentinel 2 datasets do not include thermal and Quality Assessment bands that simplify the detection of clouds avoiding erroneous classification. At the same time, also clouds shadows on the ground lead to anomalous reflectance values which have to be taken into account during the image processing.
Before GSoC 2018, only a specific module for Landsat automatic cloud coverage assessment was available within GRASS GIS (i.landsat.acca) while regarding shadows, no specific module was available.
The project creates a specific module for GRASS GIS application (i.sentinel.mask), which implements an automatic procedure for clouds and shadows detection in Sentinel 2 images. The procedure is based on an algorithm, developed within my PhD research, which allows to automatically identify clouds and their shadows applying some rules on reflectance values (values thresholds, comparisons between bands, etc.). These have been defined starting from rules found in literature and conveniently refined. In order to increase the accuracy of the final results, a control check is implemented. Clouds and shadows are spatially intersected in order to remove misclassified areas. The final outputs are two different vector maps (OGR standard formats), one for clouds and one for shadows.
More information about the algorithm and the module can be found HERE.
i.sentinel.mask is a real complete GRASS GIS addon module (with GUI and manual page) and it is now available on the official GRASS GIS SVN repository.
It can be easily installed within GRASS GIS using the module g.extension:
g.extension extension=i.sentinel operation=add
The resulting clouds and shadows maps obtained running i.sentinel.mask.
To run i.sentinel.mask, the bands of the desired Sentinel 2 images have to be imported and the atmospheric correction has to be applied.
In order to make the data preparation easier, another GRASS GIS addon module has been developed within the GSoC project.
i.sentinel.preproc is a module for the preprocessing of Sentinel 2 images (Level-1C Single Tile product), which wraps the import and the atmospheric correction using respectively two existing GRASS GIS modules, i.sentinel.import and i.atcorr.
The aim is to provide a simplified module which allows importing images and performing the atmospheric correction avoiding users to supply all the required input parameters manually. The module should help users in preparing data to use as input for i.sentinel.mask. In fact, it makes especially the atmospheric correction procedure easier and faster because it allows performing atmospheric correction of all bands of a Sentinel 2 scene with a single process and it retrieves most of the required input parameters from the image itself. Moreover, one of the possible output of i.sentinel.preproc is a text file to be used as input for i.sentinel.mask.
More information about the module can be found HERE.
i.sentinel.preproc is a real complete GRASS GIS addon module (with GUI and manual page) and it is now available on the official GRASS GIS SVN repository.
It can be easily installed within GRASS GIS using the module g.extension:
g.extension extension=i.sentinel operation=add
NOTE: the starting idea was to create a single module which wrapped all the functionalities provided by i.sentinel.mask and i.sentinel.preproc but, following the suggestions of the dev community, it was decided to develop two different modules.
i.sentinel.mask
- Implement other existing algorithm of clouds and shadows detection
i.sentinel.preproc
- Implement download functionality avoiding dependencies
- Integrate the Topographic Correction
- This pubblic repository was created specifically for the development of the GSoC 2018 project "GRASS GIS module for Sentinel-2 cloud and shadow detection" and it will obviously remain active but the development of the modules will be maintained on the GRASS GIS SVN repository in the future. Anyone who wants to contribute to the development of these two modules is invited to do it on the GRASS GIS SVN repository.
- All the file concerning i.sentinel.mask and i.sentinel.preproc have been moved to two specific subfolder in this GitHub repository. If you want to install i.sentinel.mask and/or i.sentinel.preproc you have to use the the official GRASS GIS SVN repository because g.extension for GitHub repository download currently support only module (Makefile) at the top level.