MUSCATE L2A format

 

Until July 2019, Venµs L2A products were processed at the Venµs Image Processing (VIP) center, with a specific format. From July 2019, the L2A are processed at MUSCATE with a sllghtly different format which is described here below. The reason of this change is the homogeneity of formats within the Theia platform. As a result, the Venµs L2Aformat is close to that of Sentinel-2 L2A products generated by Theia.

The main changes are:

  • the metadata format is different, the keywords have changed.
  • the images are still provided with TIFF format, but there is one file per band. The scaling factor has changed, it is now 10000, as for Sentinel-2.
  • the L2A resolution is now 5m !
  • the grid used corresponds to that of Sentinel-2, you only have to account for the upper left origin of each site to obtain the registration of the images. No reprojection and no resampling is necessary.

 

When you download a Venµs L2A product from Theia website, you obtain a zip file.

unzip -o  VENUS-XS_20190428-173944-000_L2A_ARM_D_V2-15.zip

After unzipping, you get a folder with the same name as the zipfile, without ".zip".

The main product format contains a metadata file in xml format, the surface reflectances in two flavours, the atmospheric variables computed by MAJA, and a MASK folder.

Useful information in metadata

  • the MTD_ALL.xml file contains the metadata from the image acquisition
    - Instrument, date and time
    - projection and geographic coverage
    - Solar and viewing angles
    • For the solar angles, you can use the values at the scene center.
      <Sun_Angles>
      <ZENITH_ANGLE unit="deg">24.7047221168</ZENITH_ANGLE>
      <AZIMUTH_ANGLE unit="deg">150.8701236661</AZIMUTH_ANGLE>
      </Sun_Angles>
    • For the viewing angles, you have to account for the fact that the angles differ depending on the spectral band.Venµs channels are regrouped per triplets and the viewing angles are provided for each triplet. For instance, for the third one :
      <Mean_Viewing_Incidence_Angle detector_id="03">
      <ZENITH_ANGLE unit="deg">14.813356</ZENITH_ANGLE>
      <AZIMUTH_ANGLE unit="deg">279.153377</AZIMUTH_ANGLE>
      </Mean_Viewing_Incidence_Angle>

    Images

  • the surface reflectances are provided with two flavours. There is a file for each of Venµs 12 spectral bands (XX) hereafter :
    • the files which end with "FRE_BXX.TIF" (FRE for Flat REflectance) are also corrected for slope effect, which consists in suppressing the apparent reflectances variations due to the orientation of the slopes with regard to the sun. The corrected images look like if the land was flat.

    Both types of files contain :

      - surface reflectances for spectral band XX
      - coded in 16 bits signed integers: you have to divide per 10000 to obtain reflectances
      - No_Data value (outside the image), is -10000
  • We also provide the values of the atmospheric variables computed by MACCS/MAJA, with a 5 m resolution (ATB_XS.tif)
  • The files contain two bands coded on 8 bits each :
    • Band 1 is water vapour, you have to divide by 20 to obtain g/cm2  <VAP_Quantification_Value>0.05</VAP_Quantification_Value>
    • Band 2 is the Aerosol Optical Thickness, you have to multiply by 0.005 to get the AOT. <AOT_Quantification_Value>0.005</AOT_Quantification_Value>
  • The MASK directory:

  • A no_data mask, at full resolution (EDG_XS.tif)
  • A saturated pixel mask that comes from Level 1  (SAT_XS.tif)
    • first bit corresponds to first band
    • second bit corresponds to second band
    • ...
  • An interpolated pixel mask that comes from Level 1  (PIX_XS.tif). Some pixels in the blue bands are interpolated.
    • first bit corresponds to first band
    • second bit corresponds to second band
    • ...
  • A cloud and shadows mask is provided in the file CLD.DBL.TIF:
    bit 0 (1) : all clouds except the thinnest and all shadows
    bit 1 (2) : all clouds (except the thinnest)
    bit 2 (4) : cloud shadows cast by a detected cloud
    bit 3 (8) : cloud shadows cast by a cloud outside image
    bit 4 (16) : clouds detected via mono-temporal thresholds
    bit 5 (32) : clouds detected via multi-temporal thresholds
    bit 6 (64) : thinnest clouds
    bit 7 (128) : high clouds detected by stereoscopy

    • Example 1 : value 5 = 4+1 (00000101) :  bit 0 tells us a shadow or a cloud has been detected and bit 2 that it is a shadow (4),
    • Example 2 : value 35 =  32 + 2 +1 (00100011) tells us that it is a cloud (bits 1 and 2), detected by multi-temporal threshold (bit 5)
    • How to use the cloud mask

      If you want a very strict cloud and shadows mask, just test if cloud mask is greater than 0. This is our advice.If you are working in a region where you think MACCS/MAJA detects too many clouds, check that bit 1 is 1. (Mask & (00000001) == 1)

     

  • A geophysical mask MG2 :
  • bit 0 (1) : Water mask
    bit 1 (2) : All clouds (except the thinnests)
    bit 2 (4) : Snow Mask
    bit 3 (8) : all shadows ("OU" des bits 5 et 6 du masque de nuages)
    bit 4 (16) : Topographic shadows
    bit 5 (32) : Unseen pixels due to topography
    bit 6 (64) : Sun too low for a correct terrain correction
    bit 7 (128) : Sun direction tangent to slope (inaccurate terrain correction)
  • IAB : A mask of pixels for which the WaterVapour (bit 1) and Aerosol Optical Thickness (bit 2) was interpolated
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