bul 2004 International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part Bl. Istanbul 2004 
e greater diffuse illumination effects, resulting in the BRF and DHR. 
e; as the The determination of these surface products obviously 
scattered requires that the atmosphere be sufficiently characterized in 
ing the order for the correction process to occur. This 
dn characterization is accomplished by means of an aerosol 
| (in this retrieval. After a BRF is determined, it is fitted to a three 
99817 at parameter empirical BRF model, which provides a 
scattered convenient representation of the surface scattering 
characteristics. 
um. The We statistically analyzed the differences of directional and 
from the hemispherical MISR reflectance data products, namely DHR 
otons to versus BHR and BRF versus HDRF. These data products are 
ingle. At compared to each other by their respective mean values, 
angle for mean absolute and relative difference. Additionally, their 
correlation is derived. For further analysis of the products 
and their differences, the mean value of the absolute HDRF 
uncertainty and relative BHR uncertainty product was 
calculated, as well as the mean aerosol optical depth (AOD) 
value in the green spectral band of all analyzed pixels. 
The ratio of diffuse to direct illumination increases with 
increasing AOD. Therefore we expect the largest difference 
between HDRFs and BRFs in shorter wavelength ranges, i.e., 
in the blue band, where the diffuse component of the 
illumination is largest. This wavelength dependence is due 
to the decreasing influences of Rayleigh scattering and 
aerosols with increasing wavelength. 
We selected ten datasets, acquired in 2001, that correspond 
to MISR data product version 12. For all analyzed data, a 
comparison of MISR optical depths with those from an 
included ground based AERONET site showed good 
correlation in all four MISR spectral bands. The reliability of 
the land surface reflectance values depends upon the AOD 
magnitude. Therefore, pixels with an AOD larger 0.5 at 
558 nm (green spectral band) have been excluded from the 
MISR scenes. In the following, all quantities called ‘scene- 
averaged’ rely on this exclusion. 
The sites were selected to represent different biome types, 
following the MODIS IGBP land cover map. Three sites are 
covered twice, under different atmospheric conditions and 
sun zenith angles (Table 3). 
' snow at 
  
  
  
  
  
  
  
  
  
  
  
  
  
BHR for E T Country | Date | Mean| Main biome Mean 
included 12001 | sz [°]| type (IGBP) | AOD 
FHowiend. iain c 07421 1: 07. 2 Mtl 0.10 
ducts Es US nb od __decid. broadl f... 
Railroad | Nevada, | 08/17 | 28.4 | Barren or 0.99 
ous land Volley {US dei E ood sparsely veg. | | 
he MISR Mongu | Zambia 07/11 | 44.6 | Savannas, 0.05 
th centre A ME Sn 
grees in Banizou | Niger 10/04 | 24.1 0.31 
sque 12/3 VAREL, 
i Hombori [Mali 07/05 |19.6| 1 10.36 
ie Avignon | France [07/12 [252 — 10.07 | 
im uote lent - og allo] 
ey Bordeau | France 05/30 | 24.5 | Everg. needlel. | 0.24 
ue angle. E 
i do X f., croplands, 
sction is : x 
peel ene oped aC sd id is re 
2 
products 1002/01. D.24.0 15 | 0.12 
hematical / : 
top-of- Table 3. Overview of MISR scenes selected for the analysis 
rected to of the land surface products. 
erties as 
R spatial 3.3.2 Results: Differences between BHR (Case 9) and 
:xplicitly DHR (Case 3) : 
IDRF and In general, BHR and DHR product values derived from the 
MISR sensor are highly correlated, with r^ values between 
:move all 
0.98 and 1.0 throughout all spectral bands and analyzed 
365 
scenes (with the exception of the Hombori scene blue band, 
where r^ reaches 0.84 only). 
For all analyzed MISR images, the relative scene-averaged 
difference between BHR and DHR reaches a maximum of 
2.7 % of the BHR value (with the exception of the difference 
in the blue band of the Hombori scene reaching 5.1%) for all 
four spectral bands (Table 4). Numerically, this is a small 
difference, 
compared to the data uncertainties. The lowest 
scene-averaged relative BHR uncertainty is 5.6% for the NIR 
spectral band of the Avignon (07/12) scene, whereas relative 
BHR uncertainty can easily reach values around 20% and 
much higher, with a maximum of 88% for the blue spectral 
band of the Banizoumbou (10/04) scene. 
As detailed above, we expect a trend of decreasing 
differences between BHR and DHR with increasing 
wavelength, thus the blue band reflectances should show the 
largest relative differences. Results show that the relative 
reflectance difference of five scenes is biggest in the blue 
band, whereas for the other 5 cases, differences reach the 
same or even higher values in at least one of the other bands. 
  
  
  
  
  
  
  
  
  
  
  
| Site SZ | Mean Mean BHR | 
| [] | AOD | Mean ((BHR-DHR)BHR) [%] | 
| | 446nm | 558nm 672nm| 867nm | 
| Howland | 27.7| 0.10 | 0.031 | 0.053 | 0.028 | 0.318 
| | 2l 0d. Lid 02 | 
| Railroad | 28.4 | 0.99 | 0.095 | 0.137 | 0.170 | 0.238 | 
| Valley... 1 1.7. ut 2 de 1) 
|Mongu | 44.6, 0.05 | 0.046 | 0.078 | 0.094 | 0.246 | 
lot ametial catooue sehe sand 15 0:3..1, 0:0 | 
| Banizou | 24.1 0.31 | 0.060 | 0.126 | 40:176 1..0.357 
| mbou l | La da: | fale ie 1.3 d 
| 41;4 70.110.084 |-0:160 / 0.261 1-:0.376 | 
lo: nib dirlo oed eS loce. 10e 1.06 | 
| Hombori | 19.6 | 0.36 | 0.108 | 0.232 | 0.349 | 0.412 | 
4 E | 5.1 25 1.6 i2 | 
Avignon | 25.2| 0.07 | 0.045 | 0.075 | 0.069 | 0.307 | 
| footage do ONE o9 19s | 
| 369| 0.19 | 0.050 | 0.081 | 0.079 | 0.286 | 
ee NEE SO 0.9 QTV TOR 
"Bordeaux | 24.5 | 0.24 | 0.059 | 0.097 | 0.087 | 0.320 | 
J SONNE c2 1 
24.0 | 0.12 | 0.048 | 0.078 | 0.073 | 0.304 
| 18 | 15 10 99 | 
  
  
  
  
Table 4. 
Comparison of BHR and DHR values for the 
selected MISR scenes. 
Differences between the BHR and DHR product can be related 
to the actual aerosol optical depth in the green spectral band. 
This relation is weak for the BHR-DHR differences in the 
blue band (> = 0.29) and gets much stronger with increasing 
wavelengths, with a maximum for the NIR region (r” = 0.79). 
Differences between HDRF (Case 7) and BRF (Case 1) 
As with the results for the hemispherical reflectances, the 
relationship between HDRF and BRF values show a high 
correlation, with r? values above 0.98 throughout all spectral 
bands, and view angles of all scenes (with the exception of 
the Hombori scene blue band reflectance reaching an rof 
0.67 only). 
Compared to the quantities integrated over an extrapolation 
of the view hemisphere, the relative differences of the 
reflectances of the single view angles are larger and reach up 
to 10% of the HDRF value (with the exception of the 
Hombori scene blue band reflectance difference of 14.276). 
The trend of decreasing differences with increasing 
wavelength is much stronger for the directional quantities 
than for the hemispherically integrated quantities. Thus, the