Until July 2019, Venµs L2A products were processed at the Venµs Image Processing (VIP) center. Here is a simplistic description of Venµs L2A data genrated by the VIP. distributed by Theia. From July 2019, the L2A are processed at MUSCATE with a slghtly different format.
When you download a Venµs L1C or L2A product from Theia website, you obtain a zip file.
unzip -o VENUS_20180707-182652-000_L2A_DESIP2_D_V1-0.zip
After unzipping, you get a HDR file (which is an XML file) and a DBL.DIR folder. You can go to the Format section below to see how to handle it.
- the HDR contains the meta data form 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.
</Useful_Image> </Solar_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 :
<Viewing_Angles sn="3"> <Image_Center>
- you will find inside the DBL.DIR folder (ex: VE_VM01_VSC_L2VALD_DESIP2___20180707.DBL.DIR) provides the surface reflectances with two flavours:
- the files which end with "SRE.DBL.TIF" (SRE for Surface REflectance) were corrected for atmospheric effects, including adjacency effets
- the files which end with "FRE.DBL.TIF" (FRE for Flat REflectance) are also corrected for slope effect, which consists in suppressing the apparent reflectances variations dur to the orientation of the slopes with regard to the sun. The corrected images look like if the land was flat.
Both files contain :
- Surface reflectances for the 12 channels, from B01 to B12
- Coded in 16 bits signed integers: you have to divide per 1000 to obtain reflectances
- No_Data value (outside the image), is -10000
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 is also provided, which contains the following information MSK :
- bit 0 (1) : Water mask
- bit 1 (2) : Zones masked by terrain
- bit 2 (4) : Topographic shadows
- bit 3 (8) : Sun too low for an accurate slope correction
- bit 4 (16) : Sun too tangent to surface for an accurate slope correction
We also provide the values of the atmospheric variables computed by MAJA, with a 10m resolution (ATB.DBL.TIF)
- 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>
A quality file QLT mask contains diverse information
- The first 8 bit channel is a mask that flags the saturated pixels at Level 1C for each 10m band (B2,B3,B4,B8) or each 20m band(B5,B6,B7,B8A, B11,B12). If the fourth bit is 1, it means that B8 band was saturated at level 1C.
- The second 8 bit channel is a mask that flags the bad quality pixels for each 10m band (B2,B3,B4,B8) or 20m band (B5,B6,B7,B8A, B11,B12).
- A third 8 bits channel. Only the 3 first bits are used :
- Bit 0 : is 1, for no-data pixels, outside the image footprint
- Bit 1 : tells if AOT was estimated (0) or interpolated (1)
- Bit 2 : tells if water vapour was estimated (0) or interpolated (1)