In: Wagner W., Székely, B. (eds.): ISPRS TC VII Symposium - 100 Years ISPRS, Vienna, Austria, July 5-7, 2010, IAPRS, Vol. XXXVIII, Part 7B 
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After calculating average spectra per plot, the resulting spectra 
were smoothed using a 15 nm wide moving Savitsky-Golay 
filter (applying a second order polynomial fit within the 
window) to reduce instrument noise. 
3. RESULTS AND DISCUSSION 
3.1 PROSAIL Simulations 
The simulation results obtained with the improved PROSAIL 
model (using PROSPECT-5) using a 1 nm spectral sampling 
interval showed that for many spectral positions beyond 900 nm 
the relationship between the first derivative and CWC is 
statistically significant at p < 0.001. In addition to the left slope 
of the 970 nm water absorption feature, also relationships at the 
right slope of this feature and at the left slope of the 1200 nm 
feature are highly significant (Figure 2). In this paper, focus is 
on the right slope of the 970 nm absorption feature, because 
there no influence of absorption by atmospheric water vapour is 
expected. Figure 2 shows that the reflectance at this right slope 
is increasing gradually and that the coefficient of determination 
(R 2 ) for the relationship between the first derivative of adjacent 
wavelengths and CWC is rather constant. Therefore, we may 
calculate the first derivative over a wider interval, making the 
choice of wavelengths for derivative calculation less critical and 
making the derivative calculation more robust. Experimental 
results later in this paper suggest that an interval between 1015 
and 1050 nm is a good choice. Figure 3 provides the 
relationship between the first derivative over the 1015 - 1050 
nm interval and CWC for variations in model input parameters 
as given in Table 1. There is an offset for the linear regression 
line because soil reflectance was not constant over the spectral 
interval. Field measurements at the test site yielded a reflectance 
of 0.39 at 1015 nm and a reflectance of 0.40 at 1050 nm. 
Largest scatter around the linear regression line visible in 
Figure 3 is caused by the variation in leaf inclination angle 
distribution. In the next section it will be tested whether this 
simulated relationship matches the one found with experimental 
data. 
3.2 Achterhoek Study Site 
For the Achterhoek site in total 40 plots were studied. Figure 4 
shows the R 2 -values for the relationship between spectral 
derivatives and CWC. The R 2 for the 1015 - 1050 nm interval 
again is constant for this test site. It is lower than the best value 
at the left slope, but the observed R 2 -values above 0.65 are 
statistically significant at p < 0.001. The R 2 at the right slope 
over the 1015 - 1050 nm interval is 0.68 (Figure 5). The 
predictive power of the first derivative as index for estimating 
CWC was assessed by estimating the root mean square error of 
prediction (RMSEP) using the leave-one-out cross-validation 
approach. The calculated RMSEP is 0.21 kg.m' 2 (as relative to 
an average CWC of 0.53 kg.m' 2 ). The relationship is in 
agreement with the one found for the simulated data from 
PROSAIL in Figure 3, which are plotted at the background of 
the Achterhoek results in Figure 5. 
The agreement between the experimental data and PROSAIL is 
good when using reflectance derivatives over the 1015 - 1050 
nm interval. Therefore, the relationship between first derivative 
and CWC was trained with PROSAIL and then this relationship 
was applied on the experimental data. The calibrated 
relationship is given in Figure 3. When applying this 
relationship to the experimental data of the Achterhoek site, 
Figure 2. Coefficients of determination between CWC and first 
derivative of canopy reflectance as simulated by 
PROSAIL. The dotted line provides an example of a 
simulated canopy reflectance signature. 
Figure 3. Relationship between first derivative of canopy 
reflectance over the interval 1015 - 1050 nm and 
CWC (PROSAIL simulations with varying input 
parameters according to Table 1). 
Figure 4. Coefficients of determination between CWC and first 
derivative of canopy reflectance as measured with 
the ASD FieldSpec at the Achterhoek site in 2008. 
The dotted line provides an example of a measured 
canopy reflectance signature. 
Figure 6 illustrates the comparison of the estimated values with 
those obtained from the ASD FieldSpec measurements. The 
RMSEP derived from this Figure 6 is 0.25 kg.m' 2 . This value is 
about equal to the RMSEP value of 0.21 kg.m' 2 obtained for the