3.3. GlobCover Products

The GlobCover products (pixel classification) which can be derived using the IDEPIX tool are:

MEPIX uses the implementation which had been developed within the GlobCover project.

3.3.1. Blue Bands

Table 3.4. Blue Bands derival

IDEPIX Blue Band algorithm characteristics
A simple blue band test that provides with very good results for VEGETATION has been adopted for MERIS, and enhanced it by using the 412 nm channel. The developed cloud screening method is applied to reflectances. A first threshold for the B1 reflectances (R1) is used to detect the mostbrilliant dense clouds. The cloudy pixels are validated trough a restoration process, which eliminates false detections due to snow covers. A threshold on the NDSI ((R10-R13)/(R10+R13) is used to detect snow covers. The remained clear pixels are tested by a second filter, which performs the ratio R11/R10 related to the altitude of the scattering surface. An optimised threshold permits to identify thin clouds not detect by the first blue band test. Three states are supposed: 0=Out of Orbit, 1=Clear and 2=Cloud). The optimised cloud mask is globally coherent with the bright flag of standard MERIS products. However, a higher performance of the cloud screening over semi-transparent clouds can be observed. Large areas are not detected by the bright flag or probability algorithms. This analysis shows that blue bands algorithm can better detect semi transparent clouds.


3.3.2. Cloud Probability

Table 3.5. Cloud Probability derival

IDEPIX Cloud Probability algorithm characteristics
This algorithm includes two neural networks (land/water) which get as input:
  • the reflectances in bands 1-6, 9,10, 13
  • the reflectances ratio of band 11 / band 10, which is a measure for the air mass above the measured surface
  • the surface pressure from the tie-point grid
  • the exact pixel-wavelength of band 11
  • and some pixel geometry
The output of the net is a floating number [0..1] indicating the likelihood that the pixel is a cloud. This is compared against a threshold to set a binary cloud flag