Modis Land Surface Reflectance Science Computing Facility

VALIDATION STATUS

Optimal conditions for derivation from EOS data

The accuracy of the Surface Reflectance (SR)product is mostly driven by the knowledge of aerosol optical thickness (AOT). The accuracy is better when the MODIS-derived AOT is available (Vermote et al., 1997), i.e., over areas with dense, dark vegetation (Kaufman et al.,1997). The uncertainty on AOT is expected to be (0.05+0.2*AOT) (Kaufman et al.,1997).

For the 6 different land cover types used by Vermote et al. (ATBD, 1999), (broad leaf crops, leaf forests, needle forest, grass/cereal crops, savanna and shrubland, sun zenith angle (SZA) = 30 deg.), the impact of this uncertainty on SR retrieval will lead to typical errors for AOT:

Band	AOT(550 nm)=0.1	AOT(550 nm)=0.5
Band	Absolute Error	Absolute Error
1	0.003	0.008
2	0.008	0.018
3	0.006	0.013
4	0.003	0.007
5	0.007	0.016
6	0.003	0.007
7	0.002	0.004

Other caveats

Uncertainty in the aerosol model can cause uncertainty in the atmospheric correction.

The typical errors estimated are:

Band	Absolute Error
1	0.002
2	0.017
3	0.002
4	0.005
5	0.016
6	0.009
7	0.006

AOT(550 nm)=0.3; SZA=30 deg.

The use of a Lambertian surface approximation instead of bi-directional reflectance distribution function (BRDF) can deteriorate the accuracy with larger errors in the backscattering direction (Vermote et al., 1997, Vermote, ATBD 1999): errors can be as important as 0.08 for band 2 in the backscattering direction for shrubland and a moderate aerosol loading (AOT(550 nm) = 0.3) for SZA = 30 deg. Errors are smaller in other bands (<0.02), reaching a maximum of 0.05 in the backscattering direction for bands 1, 4 and 5. Errors are much lower when an assumed BRDF is used in the atmospheric correction process. Larger errors occur in the backscattering direction and for shorter wavelengths (bands 1, 3 and 4). Maximum relative and absolute errors resulting from BRDF uncertainty (atmospheric correction over shrubland corrected assuming leaf forest BRDF) are:

Band	Relative Error	Absolute Error
1	9.0%	0.003
2	2.0%	0.011
3	8.5%	0.002
4	4.5%	0.003
5	1.5%	0.007
6	1.0%	0.003
7	1.0%	0.001

Calibration uncertainty can lead to larger errors in the shorter wavelengths (bands 1,3 and 4) and for higher view zenith angles. Typically, relative and absolute errors are not greater than:

Band	Relative Error	Absolute Error
1	4.0%	0.003
2	2.0%	0.015
3	14.0%	0.004
4	3.5%	0.004
5	2.0%	0.015
6	2.0%	0.008
7	2.0%	0.003

Moderate aerosol loading (AOT(550 nm) = 0.3), SZA = 30 deg.

Accuracy can deteriorate in polar regions, for SZA greater than 80 deg.
After a significant volcanic eruption, accuracy can deteriorate if the stratospheric and tropospheric AOT are not decoupled.
Thin cirrus contamination.
Cloud shadows.
Shadows caused by topographic effects.
Change in surface BRDF during the compositing period caused for example by locally and temporarily flooding and snow cover conditions that can partly cover vegetation (Strahler et al., ATBD 1996).

Typical Theoretical Accuracy

Band	Absolute Error	Relative Error (range)
1	0.005	10%-33%
2	0.014	03%-06%
3	0.008	50%-80%
4	0.005	05%-12%
5	0.012	03%-07%
6	0.006	02%-08%
7	0.003	02%-08%

Principal Investigator: Dr. Eric F. Vermote University of Maryland Department of Geography and NASA GSFC code 614.5	Correspondence to: 4321 Hartwick Rd. Suite 209 College Park, USA E-mail: mod09@ltdri.org
Responsible NASA Official: Ed Masouka	Web site design: mod09@ltdri.org
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