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ACCURACY & UNCERTAINTY |
VALIDATION STATUS |
ERROR BUDGET |
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VALIDATION STATUS
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Stage 1 Validation:
The results of stage 1 validation are summarized in Vermote et al.,
2002. The validation of the atmospheric correction was done partially by
validating the aerosol optical thickness used in the correction
algorithm through the comparison with AERONET data. This comparison
showed that the errors on optical thickness were distributed within the
error bars adopted to derived the accuracy statement on surface
reflectance (see Products and MOD09 ATBD).
To complete the validation process, ground measurement of the
surface reflectance were indirectly compared to MODIS using the Landsat
ETM+ as a way to achieve validation at MODIS pixel level (500m x 500m).
Landsat ETM+ data were corrected using the data from the AERONET data
base with respect to aerosol model, optical depth and water vapor, and
compared to ground measurements (performed with an ASD spectrometer) for
three different sites (yellow grass, bare soil and harvested corn site)
of about 100m x 100m. This step assured that the standard used for
validation of MODIS data (corrected ETM+ data) was itself valid.
The ETM+ data were then averaged to the 500m MODIS resolution and
only more homogeneous pixels with a low standard deviation were retained
for comparison to MODIS pixels. It was shown that the MODIS operational
product was definitely within the error bars announced in the ATBD for a
wide range of reflectance values.
Stage 2 Validation:
Validation at stage 2 is ongoing. Over 4988 samples
of Terra data were compared over 150 AERONET sites in 2003. Using an
error bar of +/- (0.005+5%) for reflectance bands, we have shown
that for good quality retrievals (QA: no cloud, cloud shadow or high
aerosol), 86.62% of the observations for Band 1 and 94.13% of the
observations for Band 2 were within the error bars. Using an error
bar of +/- (0.02+2%) for Vegetation Indices, we have shown that for
good quality retrievals (QA: no cloud, cloud shadow or high
aerosol), 97.11% of the NDVI observations and 93.64% of the EVI
observations were within the error bars. |
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