Update README.md

README.md

@@ -16,3 +16,30 @@ The figure below shows the layout of the file and some sample data:
 ![hdf5 file](fig/screenshot_hdfview.png)
 
 A technical note on how the data was produced is currently under preparation.
+
+## How the data was produced
+
+### 1. Data Collection
+
+Open-source Sentinel-2 data is available for download on the [Scientific Data Hub](https://scihub.copernicus.eu/dhus). Products consist of a 290 km image divided into 100 km granules in UTM/WGS84 projection. The product name includes sensing and creation date, as well as the relative orbit number of the image.
+Following image corresponds to the division into granules of the product **S2A_OPER_PRD_MSIL1C_PDMC_20151211T153317_R021_V20151211T084342_20151211T084342.SAFE**:
+
+![granules](fig/screenshot_granules.jpg)
+
+To create a representative spatial dataset, downloaded images cover a high variety of regions from all over the world. 
+
+### 2. Data Classification
+
+By means of different spectral tools, granule pixels are selected and classified into one of the following six classes: 
+- cloud
+- cirrus
+- snow
+- water
+- clear-sky
+
+Our dataset consist of a total of N=5647725 pixels. Pixel information is saved into different tables. 
+- **classes**: 1xN table including the class id to which each pixel in the dataset is associated
+- **class_ids** describes the ID associated to each class in **class_names**
+- **spectra**: 13xN table with the spectral values of each pixel. Sentinel-2 instrument samples 13 spectral bands.
+- metadata can be accessed in group *metadata*
+- Sentinel-2 band information under *bands info*
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