April, despite a certain amount of cloudiness, once again offered us a beautiful, cloudless synthesis. In fact, Theia's Level 3A products use 46 days periods centered on the 15th of the month, and the good weather of the last days of March or the first days of May compensates the cloudy April. As every month, Peter Kettig from CNES produced the Level 3A syntheses from the previous month's Sentinel-2 data. As you can see, a column of tiles is degraded in western France. This is due to a problem during the download of Sentinel data by PEPS. We're going to reprocess it.
Translated with www.DeepL.com/Translator
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 have a look to the mosaic of Sentinel-2 monthly syntheses for each month since July over France. Each monthly synthesis is accessible using the following links :
Or you may also use the nice viewer below (merci Michel Lepage !) to compare with the previous months.
See it full screen
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.
The monthly syntheses are produced using the WASP processor, which is described here.
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.