On June 23 we will celebrate the third anniversary of Sentinel-2A in orbit. With three years of data we can start looking at the inter-annual variability of biophysical variables, like.. (random example), the snow cover.
This is what I attempted to do for the Theia workshop. I downloaded all available snow cover products from Theia over the Central Pyrenees (tile 31TCH) and I generated additional snow maps from the Theia Landsat-8 level-2A products using let-it-snow processor. Landsat-8 images enable to increase the frequency of observations when only Sentinel-2A was operational between 2015 to 2017.
I resampled the Landsat-8 snow maps to the same reference grid as Sentinel-2 at 20 m resolution using the nearest neighbor method. I cropped all snow maps to the intersection of the Sentinel-2 tile (green polygon) and Landsat-8 tile (red polygon).
When there was a snow map from Sentinel-2 (S2) and Landsat-8 (L8) on the same day, I merged them into a composite using a simple pixel-based rule:
Le produit vecteur d'OSO 2017 est enfin sorti ! Après plusieurs semaines de traitements, les vecteurs de chaque département sont disponibles ici. La production requiert la mobilisation d'une grande quantité de ressources de calcul et une stratégie de traitements un peu particulière. Nous voulions vous expliquer comment parvient-on à produire cette couche d'information.
Exemple du raster initial (10 m), régularisé (20m) et vectorisé
A priori, le plus simple serait de prendre la couche raster issue de la chaine de traitements iota² de l'intégrer dans notre logiciel SIG préféré et d'appuyer sur le bouton "Vectorisation" ! Mais les choses ne sont pas si simples, certaines contraintes et besoins nous obligent à quelques tours de passe-passe :
We have just updated the MAJA/THEIA workshop website to add a draft program. The workshop will be held in Toulouse, from 13th to 14th of June, and will be hosted by the ENSEEIHT engineering school in Toulouse historical center. Registrations are still open, until the third of June.
The aim of this workshop is to collect feedback and share experiences on the quality, use and applications of the L2A surface reflectance products delivered by Theia from Sentinel-2 data.
The meeting objectives are as follows :
- to provide information about L2A product status and validation
- to gather feedback from users about L2A product quality
- to show applications and results of L2A Sentinel-2 time series
- to share experiences on how to use the products
- to collect suggestions for improvements.
We look forward to meeting you soon !
The organising Comittee (Arnaud Sellé, Olivier Hagolle, Céline Arnal)
Atmospheric absorption: in blue, the surface reflectance of a vegetation pixel, as a function of wavelength. In red, the reflectance of the same pixel at the top of atmosphere. For a wavelength of 1.38 µm, water vapour totally absorbs the light that comes from the earth surface at sea level. At 0.94 µm (940nm), a weaker water vapour absorption band only partly absorbs the photons.
Sentinel-2B has two channels centered on water vapour absorption bands: channel 9 (940 nm) and channel 10 (1380 nm). Band 10 corresponds to a very strong absorption, strong enough to prevent any photon to reach ground from the Sun without being absorbed in the atmosphere. This band is intensively used to detect and correct high clouds.
In this blog, we discussed much less band 9 (940 nm) yet. Here, water vapour absorption is not strong enough to catch all the photons which reach the surface. The proportion of absorbed photons depends on the water vapour atmospheric content, and also on the viewing and solar zenith angles. We use band 9 for atmospheric correction, but it could be useful to study convection phenomena within the atmosphere too.
Example of cirrus cloud correction
We will start distributing MAJA V3.1 this May to replace MAJA V1 on CNES free software platform.
It is also in the pipeline of enhancements of Theia processing platform (MUSCATE), but this pipeline is quite full, so we will need to be patient (which requires a big effort for me, patience not being my best quality...)
MAJA V3 comes with a lot of enhancements compared to V1 :
Did you know that the St. Moritz Casino is the highest in Switzerland? If you like gambling, I have a little game for you: what are the odds to find snow near St. Moritz?
Fortunately, I just finished the processing of 218 Sentinel-2 dates from 2015-Dec-04 to 2018-Apr-10 of tile 32TNS with our let-it-snow processor. I did this off-line production for a colleague because, as of today, Theia only distributes the snow products after July 2017 in this region of Switzerland (see the available products here).
A quick way to check the output is to compute a snow cover probability map: that is, for each pixel, the number of times that snow was observed divided by the number of times that the snow could be observed.
To compute this map we just need to know that the Theia snow products (LIS_SEB.TIF raster files) are coded as follows:
205: Cloud including cloud shadow
254: No data
Here is a piece of script to do this:
# initialize snow.tif with zeros
# store in Byte because we have less than 255 dates
f0=$(find . -name LIS_SEB.TIF | head -1)
gdal_calc.py --overwrite -A $f0 --type=Byte --calc=A*0 --outfile=snow.tif
# accumulate snow pixels in snow.tif
for f in $(find . -name LIS_SEB.TIF)
# snow is coded with 100
gdal_calc.py --overwrite -A $f -B snow.tif --type=Byte --calc="B+(A==100)" --outfile=snow.tif
# now do the same for clear.tif
gdal_calc.py --overwrite -A $f0 --type=Byte --calc=A*0 --outfile=clear.tif
# accumulate clear pixels in clear.tif
for f in $(find . -name LIS_SEB.TIF)
# only snow and no snow are coded with values lower than 101
gdal_calc.py --overwrite -A $f -B clear.tif --type=Byte --calc="B+(A<101)" --outfile=clear.tif
# Finally compute the snow probability in % (100.0* makes the calculation in float)
gdal_calc.py -A snow.tif -B clear.tif --type=Byte --calc="(100.0*A)/B" --outfile=snowProba.tif
This is the output:
The images are scaled from 0 (black) to 100 (white). The units are number of days for snow and clear, percentage for snowProba.
From which you can map the odds to find snow near St. Moritz (click on the image to animate)!
MUSCATE sentinel-2 L2A counter just reached 80000 products, but a lot more products were processed these past weeks. Indeed, the MUSCATE team started processing the early Sentinel-2B images acquired from July to October 2017, which were missing on our catalog. Indeed. We only started Level 2A treatments in October 2017, in real time (a bit late, but before the ESA;)). We take advantage of this processing to reprocess Sentinel-2A data, since the quality of the products from MAJA improves with the repetitiveness of the observations. And since we replace old versions with new ones (v 1.7), the product counter is not affected by the reprocessing of S2A data.
The reprocessing of data on France and its overseas territories is finished, and the treatment of the other European zones is in progress. We will continue soon with the African zones then those of the rest of the world. If you are using the Sentinel-2 data acquired in the second half of 2017, we encourage you to download them again. We took advantage of this reprocessing to also produce snow products in the France area.
Le compteur de produits de Niveau 2A de Sentinel-2 de MUSCATE vient de franchir les 80000 produits, mais en fait, MUSCATE en a produit beaucoup plus. En effet, l'équipe d'exploitation (Joel, Raquel, Sylvie) a commencé à traiter les données de Sentinel-2B acquises de juillet à octobre 2017, qui manquaient dans notre catalogue. Nous n'avions démarré les traitements de niveau 2A qu'en octobre 2017, en temps réel (avec un peu de retard, mais avant l'ESA ). Nous profitons de ce traitement pour retraiter les données Sentinel-2A, puisque la qualité des produits issus de MAJA s'améliore avec la répétitivité des observations. Et comme nous remplaçons les anciennes versions par les nouvelles (v 1.7), le compteur de produits n'est pas affecté par le retraitement des données S2A.
Le retraitement des données sur la France et ses territoires d'outre-mer est terminé, et le traitement des autres zones Européennes est en cours. Nous poursuivrons prochainement avec les zones Africaines puis celles du reste du monde. Si vous utilisez les données Sentinel-2 acquises au deuxième semestre 2017, nous vous encourageons donc a les télécharger à nouveau. Nous avons profité de ce retraitement pour produire également les produits neige sur la zone France.
Great news for our little Venµs satellite project: Theia has started distributing Venµs L1C data in near real time. The data acquired yesterday are already available on https://theia.cnes.fr
These L1C products are tagged with 0.9 version, which means that their quality is not as good as what we think we will be able to deliver in a couple of months. The multi-spectral and multi-temporal registration can still be improved, even if it is not too far from our expectations yet in most cases. The image quality teams are finishing an error budget of the current situation, and also preparing the improvement with our Israeli partner who manage the satellite. We will tell you about that.
The L2A delivery will also start in a few weeks, but the L1C teams, who had much more work than expected with this satellite, handled us correct products very late, and we still need to tune a few parameters to provide good quality products.
On the Theia website, you will find several ways to download the tiles. My little download tool has also been updated. Once you have registered, and updated the config.cfg file, you will be able to download at once all the products in Australia with the following command line :
python theia_download.py -l 'Australia' -c VENUS -a config.cfg -d 2018-01-01
The data format is explained here. Its packaging with useless zips and tars is still provisional and will be simplified very soon.
A first set of 35 images (L1C) is available for download on the following CNES web site:
Venµs is now systematically acquiring data on all selected sites since January 2018, although a few acquisitions were missed from time to time for various technical reasons, which should become progressively less frequent.
The commissioning phase of the Venµs mission is not yet completed regarding image quality (radiometry and geometry). This delay is due to several issues, the most important of which are detailed hereafter. However, since these issues do not prevent the use of the data, CNES decided to process and distribute a first set of 35 Level-1 (L1) images for which the radiometric and geometric qualities are acceptable. Level 1 corresponds to orthorectified Top of the Atmosphere reflectances. This preliminary data set is intended to allow the users to familiarize with the data and their format.
These 35 images are only a subset of the data that are systematically acquired over the Venµs sites since January 2018. We plan to start the distribution of these L1 Venµs time series by April 2018. We expect to start the distribution of the Level-2 products (surface reflectances) by late May or beginning of June, 2018.
- The absolute calibrations of the spectral bands B1 (415 nm) and B2 (440 nm), both located in the blue, are less accurate than the calibration of the other spectral bands. This issue prevents the use of B1 and B2 for retrieving the aerosol optical depth (AOD) or for water color applications.
- The accuracy of the satellite attitude restitution is for now lower than expected. Due to the fact that the different bands are not acquired simultaneously this has mainly impacts on the inter-band registration. For some sites with heavy cloud cover or uniform landscape, this issue also impacts the multi-temporal registration.
The characterization of radiometric and geometric performances is still ongoing. Efforts are also devoted to the improvement of the preprocessing algorithms.
As soon as significant progresses are made, the whole data set acquired since January 2018 will be reprocessed with the new parameters and algorithms.
The product format document is available here. A more complete version will soon be made available there.
The Venµs page on the Theia web site will keep you informed on the progress made.
Time series of Venµs images acquired over Oklahoma (USA) and processed at level 2A (with a first try of atmospheric effects correction applied)