Impact toolbox. ZIP/DN to TOA reflectance. Principles and tutorial

Transcription

1 Impact toolbox ZIP/DN to TOA reflectance Principles and tutorial

2 ZIP/DN to TOA reflectance principles RapidEye, Landsat and Sentinel 2 are distributed by their owner in a specific format. The file itself has a specific file format and the data products consist of quantized and calibrated scaled Digital Numbers (DN). The role of Impact ZIP/DN to TOA import tool is to: - Transform the file format into a single GeoTiff image. Geotiff are georeferenced image file format, supported by all GIS software. After importing with Impact, you can re-use the Geotiff with other software; - Rescale DN values to obtain Top Of Atmosphere reflectance, using radiometric rescaling coefficients provided in the product metadata file. To use the ZIP/DN to TOA reflectance function, Impact requires that you copy your original Landsat/Sentinel-2/RapidEye zipped file in the DATA/DATA_RAW directory

3 Geo Tiff conversion Impact exports the images into a single GeoTiff multi-band image. All bands are resampled to the same spatial resolution. For example, if you choose 10m for the output resolution for a Sentinel 2 image, the 60m and 20m bands are resampled to 10m. The file naming is automatic and applies the following method: Landsat: Multispectral GeoTiff file: [sensor]_[path]_[row]_[ddmmyyyy].tif Landsat format also generates different meta-information files, with same filename and a range of extensions (.met,.txt, ). Please do not delete those files. Sentinel 2: ESA supports different filenaming convention. JRC s CID portals uses ESA s safe name: S2A_MSIL1C_YYYYMMDDT[acquisition time]_nnnnn_r[nnn]_t[grsm]_[processing date and time] Other similar naming convention from ESA: RapidEye: the output keeps the filename defined in the original input.

4 TOA Reflectance Conversion (Landsat and RapidEye) The raw DN data stores the radiance measured by the satellite sensors, rescaled to fit either on 8 bits or 16bits, depending on each sensor calibration parameter (each band has a specific gain and offset). DN values are first rescaled to their original radiance values. Then, each radiance L λ are converted into reflectance R λ, which is the ratio between the irradiance received from the Sun and the radiance measured by the sensor, for a given band λ. We talk about the reflectance at the Top Of the Atmosphere R TOA λ, as no atmospheric correction is applied (this would require to know a series of atmospheric parameters at the date and time of the image acquisition): R TOA λ=π L λ d 2 E SUN λ cos(q l ) where E SUN λ is the mean solar exo-atmospheric irradiance, q l the solar zenith angle and d the earth sun distance for the date of the image; these 2 last parameters being deduced from the image meta-information. If you choose to apply a calibration and reflectance processing, the filename will show calrefl at the end. Sentinel 2 data are provided in reflectances

5 Tutorial #1 This tutorial shows how to ingest a Sentinel2 image and obtain a multi band Geotiff Top Of Atmosphere reflectance file.

6 Obtain a zipped raster image, for example from the CID portal For more details, see our tutorial Getting Landsat and Sentinel 2 data Copy the image into Impact/DATA/RAW_DATA. Click on Sentinel2 logo in ZIP/DN to TOA reflectance

7 The interface lets you select the zipped image to process. Be sure to have your original zipped file in impact\data\raw In the Processing options section, you must define: 1) The bands to import. By default, Impact preselect bands that are similar to Landsat8 bands; 2) The projection (EPSG:4326, UTM or another one from the predefined listm or directly type an EPSG code); 3) The spatial resolution; 4) The output type; 5) The Area Of Interest (optional) to clip. To use this option, you must first have loaded a polygon in Impact; 6) The output name convention. If you already have run the processing on the same input file, be sure to click on «Overwrite Output: Yes»

8 Result Filename is: original archive filenname + TOA

9 Tutorial #2 This tutorial shows how to ingest a Landsat image and obtain a multiple band Geotiff

10 Obtain an original Landsat raster image, for example from our tutorial server. Copy the image into Impact/DATA/RAW_DATA. Click on LANDSAT logo in ZIP/DN to TOA reflectance

11 The «processing type» section offers 3 choices: - Keep values as they are (keep DN values) - Transform DN to TOA reflectance, which implies applying calibrating factors and then the equation to transform radiances into reflectance, and then rescale the result to fit in a Byte (0-255) - Same as above, but save the reflectance as floating point values (values between 0 and 1). Note: most Landsat scenes are provided as DN. However, NASA is also providing Top of Canopy (Bottom of the Atmosphere) surface reflectance values. Those filenames end with _sr_. In this case, be sure to select keep DN. By default, the date is written with the format DDMMYYYY in the filename, to ensure backward compatibility with older applications. However, you may prefer to have data formatted YYYYMMDD, which is better suited for filename sorting.