October 28, 2014

Level 3 product

Simulation of level 3 products with Formosat-2

Tensift data set (Morocco)
(animation at better resolution)
Muret data set (Southwest France)
(animation at better resolution)
These Images are simulations of Venµs level 3 products, from Formosat-2 data sets. These data are orthorectified, registered, calibrated and converted to top of atmosphere reflectance at level 1, then a cloud screening and an atmospheric correction are performed. Then, for each 15 days period, an average of the cloud free surface reflectances is computed. Some pixels are kept black since no cloud free pixel was available during the compositing window. Formosat 2 images were acquired nominally every third or fourth day, but with frequent data gaps. For this reason the compositing period was longer than the anticipated value for Venµs.

Description of level 3 product

Product Content

The aim of Venµs level 3 product is to reduce the data volume for users: the idea is to provide every 7 or 10 days, a synthesis products that provides cloud free surface reflectances (as far as possible) based on the level 2 data gathered during a short period of time.

The level 3 is obtained by composition of the surface reflectances of cloud free pixels acquired during the compositing window; the compositing window might be a little longer than the compositing period in order to maximise the probablility of obtaining cloud free pixels. The reflectance of level 3 pixels will be computed using one of these two methods (probably the first one):

  • Weighted Average of the cloud free reflectances. The weights can include a quality index of the atmospheric correction for level 2 products.
  • Selection of the best cloud free reflectance during the period, with for instance a choice of the maximum NDVI or minimum of blue band.

Of course, the level 3 product is accurate only if the surface reflectance does not change within the compositing time window. For instance, if a parcel is ploughed within the compositing time window, the level 3 product will average very different surface reflectances, whereas the Maximum NDVI will select the reflectances acquired before the ploughing.

Algorithm description