First Level 2A time series of SPOT4 (Take5) images

(aerosol images have been added at the end of the post)

The verification of the various steps of our SPOT4(take5) processing scheme is going on. On Thursday, we received our first time series, I orthorectified them on Friday, and we were then able to start testing our level 2A processor with the first time series. The one displayed below was obtained on the CESBIO site in Tensift valley : Marrakech is near the center of the image, while the Atlas mountains are in the South East part of the image.

The images on the left column are ortho-rectified, and expressed in Top of Atmosphere reflectance (Level 1C product), while the right column displays the same images after atmospheric correction and cloud detection (Level 2A products), produced by Mireille Huc (CESBIO).

We quickly figured out that the cloud detection would be easy on these very clear images, even if on the February 10th, several diffuse plane contrails can be hardly seen but are partially detected, and some of their shadows as well (clouds are circled by red lines, while shadows are circled by a black line). No false cloud detection is visible. Water bodies and snow are also correctly detected for this first try (circled in blue and purple respectively)

The atmospheric correction, based on a multi-temporal method that detects the aerosols, enabled to detect that the image of February the 5th was hazier than the images of January 31st and February 10th.The February 5th image (left column) has a subtle blueish haze compared to the other dates. On the right column, the tint is roughly constant from one image to the other, which means that the aerosol detection and the atmospheric correction are working well. The aerosol images provided below are also very consistent, with the Atlas mountains playing their role of physical barrier blocking the aerosols on either side of the images. There is an aerosol measurement station on this site but it broke down at the end of January, just for the start of the experiment : Murphy's law...

So, we have reviewed and tested all the steps of the processing, but we still have to check that our methods are sufficiently robust to handle correctly the very diverse situations offered by the 42 sites. How do you say, in English "ce n'est pas une mince affaire" ?

Level 1C products expressed in reflectance at the top of atmosphere.
(c) CNES, processing : CESBIO
Level 2A products expressed in surface reflectance after atmospheric correction
(c) CNES, processing : CESBIO

Aerosol optical thickness images are displayed below. One can note that the image of the February 5th is consitent with a lot of aerosols in the North of the Atlas, and nearly no aerosols in the South. The mountains often act as barriers for the aerosols witch usually stay at a low altitude. The orange dots correspond to the snow mask whereas the red ones correspond to the cloud mask. The brighter spots on the last image may be artifacts.

Posted under: Corrections (geometry, atmosphere, clouds), In English, SPOT4 (Take 5)

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