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In: Wagner W., Székely, B. (eds.): ISPRS TC VII Symposium - 100 Years ISPRS, Vienna, Austria, July 5-7, 2010, IAPRS, Vol. XXXVIII, Part 7B 
followed by the calculation of the broadband shortwave albedos 
from the generated reflectance data. In order to avoid confusion 
the terms MODIS s ; m and TM sim will be used in the following to 
define the data that resulted from the spectral and spatial 
processing described above. 
Real Data 
From the MODIS scenes, subsets were defined to cover the 
relatively small study area. The “MODIS reprojection tool” 
(MRT) was used to trim the data to the desirable extent and to 
adapt the geodetic projection systems. 
To allow a quantitative comparison of the albedo values 
received from MODIS and TM, a systematic degradation of the 
TM data was performed (Justice et al., 1989; Kavzoglu, 2004) 
using the PSF of MODIS (derived from a series of prelaunch 
measurements at system level; Barnes et ah, 1998). 
The normalized PSF of each MODIS band was applied to the 
corresponding TM band. As a final degradation step, TM data 
were aggregated to a pixel size of 463m, which corresponds to 
the MOD09 product. In the following, broadband albedos 
calculated from MOD09 and the aggregated TM data (TM agg ) 
were compared for all acquisition dates. 
Simulated Spectral Response Function MODIS Band 1 
Figure 2. Example of using HyMap data to generate ideal 
MODIS data: HyMap channels 13-17 (Gaussian response 
functions) were weighted according to the MODIS spectral 
response function to generate MODIS band 1 
4. RESULTS & DISCUSSION 
Simulated Data 
The comparison of albedos derived from MODIS S i m and TM sim 
showed an almost perfect linear relationship (Figure 3). Values 
scattered marginally around the 1:1 -line at the highest range of 
values, but in total, deviations from the 1:1-line were 
negligible. Pearson’s r was 0.996, and the root mean squared 
deviation (RMSD) equalled 0.006 (Figure 3). The conversion 
formulae have been provided by Liang (2000) to cover a wide 
range of natural surface types. The almost perfect match 
between both datasets found here verified these formulae that 
obviously can be applied consistently for surface characteristics 
found in the central European study region with a hetero 
geneous mixture of different cover types in one pixel. This 
again suggests that a linear mixing principle is valid for 
heterogeneous (mixed) surface types (Liang et al., 2002). As 
the results of the applied processing scheme to degrade TM 
data to broad MODIS-like pixels were consistent, this approach 
was applied without modification to the real TM data. 
Shortwave Albedo 
Figure 3. Comparison of shortwave albedo derived from 
MODIS sim and from TM sim after aggregation 
Real Data 
For the real data acquired at 28 May 2005, the albedos retrieved 
from the degraded and aggregated TM data correlated highly 
with those from the MOD09 product (Pearson's r = 0.88) and 
grouped around the 1:1-line (Figure 4), although viewing 
geometries differed significantly between both sensors (see 
Figure 1 for MODIS, TM with near-nadir viewing geometry). 
Daily albedos (MOD09) were also highly correlated with the 
16-day composite, but with an offset of about 0.03 (MOD09 > 
MCD43A3). Different from MOD09 and TM data, MCD43A3 
reproduces the 16-day average condition. It also encloses a 
correction of the surface BRDF effects. 
Results were different for 19 July and 4 August 2003. Again, 
linear relations between albedos from MOD09 and TM agg were 
found, r equalled 0.94 and 0.86, respectively, but at both dates 
albedos from TM agg had a clear offset towards MOD09 results 
(TMagg > MOD09). One explanation for these results differing 
from 2005 might be, that effects from BRDF changed distinctly 
from July/August to May (mainly due to phenology). As for 
example coniferous forests with relatively small phenological 
variations were also included in the analysis, this yields not a 
full explanation. MOD09 and MCD43A3 showed a very good 
match at both dates in 2003 with low deviations around the 1:1- 
line (19 July 2003: r = 0.9751; 4 August 2003: r = 0.9121). The 
mismatch between TM albedos and MCD43A3 is not in line 
with the findings of Liang et al. (2002), who stated a very good 
agreement of total shortwave albedos retrieved from these data. 
Nevertheless, Fang et al. (2004) also found some moderate 
mimatches between albedos obtained from MODIS and Landsat 
ETM+. For these deviations, uncertainties associated with the 
atmospheric corrections (water vapor content and aerosol 
corrections) may provide some explanation (Liang et al., 2002). 
5. CONCLUSIONS 
From our findings, the following main two conclusions may be 
drawn: 
- The formulae of Liang (2000) to calculate land surface 
broadband albedos from TM and MODIS data were 
successfully applied to specifically degraded HyMap data. 
Conversion formulae were suitable for mixed surfaces of 
different land cover types. 
L