The Sentinel-2 Level 3A syntheses for September have just been made available on the distribution site of Theia products at CNES. You will find there not only the syntheses for France, but also those for Spain and Italy. We have also added the New Caledonia islands. As the generation of mosaics takes time, and as I lack time a lot, I started by publishing the mosaics above France, the other regions should come soon.
France did not change much between August and September 2019, and there is barely a slight regreening in the northern third of France. Compared to this year, the autumn drought of 2018 was even worse, while in central (Allier) and south-eastern France, the situation worsened in 2019. You can click on the animations to compare, or go to the comparator provided below, to observe it at maximum resolution.
|Comparaison of the syntheses of September 2018 and 2019||Comparaison of the syntheses from July to September 2019|
Since July 2018, the Level-3A syntheses are automatically produced by MUSCATE . Well, as always, everything was not straightforward in this production, but the exploitation managed to get the production done, and we are starting to extend it to other countries. L3A data have been produced over Spain in August, and production will be extended to Italy next year.
The full resolution data, and the corresponding data quality masks, can be downloaded from Theia's distribution server at CNES.
If you are not afraid to spend too much time while you have urgent things to do, you may also use the nice viewer below (merci Michel Lepage !) to compare with the previous months :
Here how to cite this data set :
made from Sentinel-2 images from the Copernicus program of the European Union
Processed by CNES for Theia data Center, based on methods developped ar CESBIO
In September 2019, there are relatively few changes compared to August, so drought still prevails. The far west of Brittany remained cloudy with each passage of Sentinel2, and good observers may have noticed some of the tile boundaries that have become visible, some investigation work in perspective to fully understand their origin. It can also be noted, when comparing compared to 2018, that there are still far fewer névés in the Pyrenees than last year, which had an exceptional snowfall. In fact, most of the névés seem to have disappeared.
In August 2019, most France continued to dry up, although some regions in the South of France received water thanks to the summer thunderstorms, and greened a little. Some artefacts along columns of tiles can be seen, and are related to missing products in PEPS, where we get Sentinel-2 L1C data.
In July 2019, with the exception of Brittany and southern Aquitaine, 2019 drought and heat waves have almost turned all France in to brown, particularly in the northern part of the Massif Central.
In June 2019, because of a little design error in the integration of WASP within MUSCATE, the exploitation team had to reduce the composite period to 30 days instead of 46. This synthesis will be reproduced when the error is corrected. Due to that, the remaining cloud cover is greater, and around Toulouse, there are no cloud free data, despite the area was mostly cloud free except every 5th day (bad luck).
In May 2019, The cloud cover in may was quite high, and the synthesis keeps several cloudy pixels (which are flagged as such). The column of tiles along the Greenwich meridian still suffers from missing L1C data, and is therefore degraded, with a loss of continuity with the adjacent tiles, and sometimes data gaps.
In April 2019, the weather was very cloudy, with a few spells of sunshine, and good weather at the end of March and beginning of May, which made it possible to produce a cloudless synthesis over 46 days, centred on April 15. Except on a column of tiles, where PEPS seems to have forgotten to retrieve half of the data from ESA servers, I hardly saw any artifacts. We will of course run again the L2A and the syntheses. Snow cover has been reduced, but modestly, due to late snowfall this year. On the other hand, the greening of crops, grasslands and forests is very clear. Like in the song my children learned in kindergarten, "houli houlà, le printemps est là" (spring is here) !
In March 2019 the weather remained nice and warm, and we were beginning to talk about drought. Fortunately for farmers, and unfortunately for the monthly syntheses, April was much more cloudy and rainy. On the March images, I found almost no artifacts, except on water and around the edges of snowy areas. There may be a visible swath edge between Paris and Brittany, but this is not obvious when zooming in. Snow cover has greatly reduced, particularly on the Massif Central, and crops have begun to turn green, especially in the South, much less in Alsace. The deciduous forests wisely waited until April to put their leaves on.
In February 2019 the weather was nice, hot and dry. Sentinel-2 was able to acquire several cloud free images in nearly all regions In February, snow is abundant, winter crops are green, and deciduous forests are brown. That's a normal february image, except for the clouds. Speaking of clouds, a low cloud escaped the detection within MAJA just North of Bourges. A good reminder to go on improving our methods. MAJA 3.3 should improve the results, and it's nearly ready. If you zoom on the snow covered regions, you will see atefacts, which come from WASP. We know how to improve them, we just need to find some time. And finally, because of the low sun elevation, directional effects are increased and our correction model is not perfect. Some swath edges are visible in the West of France. But all in all, it is a good February synthesis.
In November, in France, we had a... French November weather, and several zones stayed overcast for all Sentinel-2 overpasses during the synthesis period of 45 days. In that case, we try to provide a value, which is the minimum reflectance in the blue band. Of course this value is flagged as invalid. So the November synthesis is not as nice as the previous ones, due to the presence of remaining clouds. As in October (see below), we now also see artefacts at the edges of the swath. Anyway, in many regions, the results are rather correct and they allow us to see the changes. Forests are now brown, soils are wetter and darker, winter crops have started, and the highest mountains are turning white.
In October, we had the first the opportunity to observe a neat swath edge effect in four months, near Cambrai, North of France. The Western part of the artefact is browner than the Eastern part. Because of the cloud cover, the average date used in the eastern part is several days before the average date of the western part. , due to the observation at very different dates on each side of the swath. So the only way to improve that with the current method would be to add a third or even a fourth Sentinel satellite.
Methodology, caveats and artefacts
The monthly syntheses are produced using the WASP processor, which is described here.. In a few words, it computes a weighted average of the cloud free surface reflectances from L2A products processed with MAJA. It is therefore very sensitive to the quality of cloud detection (which is good, thankfully).
By comparing the various syntheses, you will see the evolution of the landscape, generally a little greener in October, but this representation will also help you spot the composite artefacts. These are not very numerous, but you will see them :
- on some web browsers (firefox V58), geometrical differences appear even at a low resolution. Other browsers and versions do not have this defect. It is really not due to Sentinel-2 or Theia products
- above water and snow (we must work on this defect)
- where clouds have covered a place during the whole synthesis period. These pixels are flagged as invalid in the products (but not on the mosaic), and for the mosaics, we provide the minimum of blue value.
- where clouds or shadows were not properly detected by MAJA
- at the edges of Sentinel-2 swath (but the effect is really faint)
- some tile edges in July, due to the fact that Level 3A products were not all generated for the 15th of July, but for dates between the 8th and the 26th. This has been corrected for August and September.
Direct acces to monthly mosaics
Each monthly synthesis is accessible using the following links :
- July 2018
- August 2018
- September 2018
- October 2018
- November 2018
- February 2019
- March 2019
- April 2019
- May 2019
- June 2019
- July 2019
- July 2020