The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008 
chlorophyll concentration is restricted to the visible region and 
the red edge, with greatest SD (2.69) around 710 nm. Next, to 
gain insights in the dynamics of the chlorophyll-generated 
variability, the Coefficient of Variation (CV) was calculated: 
CV.«=-, (1) 
The CV Cab is a useful statistic for comparing the degree of 
dispersion, from one data series to another, even if the means 
are drastically different from each other. Figure lb shows the 
CV C ab of th e PROSPECT-generated spectra with varying 
chlorophyll content. The derived CV Cab is characterized by a 
peak in the red edge at 710 nm, which is the response of 
chlorophyll a absorption. 
wavelength [nm] wavelength [nm] 
Figure 1. a) Averaged reflectance of a PROSPECT-simulated 
needle, understory and bark representing the spectral properties 
of PV, background and NPV relevant for FLIGHT 
parameterization. The grey band represents the interpolated SD 
related to the Cab range, b) associated CVCab 
2.4 Generation of FLIGHT data set 
Variable 
Unit 
Generic 
field 
observations 
Range of variation 
LUT 
Min Max ste 
P 
Within-crown 
NPV-PV 3 
% 
0.7 
0.2 
1.0 
0.1 
Crown LAI 
- 
2.5 
1 
10 
0.5 
b 
CC 
% 
0.6 
0.2 
0.80 
0.1 
LAD 
Spherical 
Stand 
structure 
Tree height 
m 
11.93 ±2.9 
Crown radius 
m 
0.88 
Trunk height 
m 
7.0 
Trunk 
m 
0.179 (at 
diameter 
ground) 
Table 1. Ranges of within-crown structural variables for 
generation of LUT and field observations of stand variables 
relevant for FLIGHT parameterization (a: NPV=1-PV; b: 0.5 
until LAI: 5 then steps ofl.O.). LAD: leaf angle distribution. 
Having introduced spectral variability at needle level, the 
analysis shifts to canopy level through coupling with FLIGHT. 
Automatic simulations were realized based on Look Up Tables 
(LUT). Prior to configuring the look-up tables, it is of 
importance that the relationship between confounding factors 
(e.g. structure, woody elements) and chlorophyll content is 
established for any given canopy structure or composition that 
may occur during a development phase. Key biophysical 
components that vary throughout stand development were 
selected, being canopy LAI, crown coverage (CC) and, to 
accommodate for a varying canopy composition, within-crown 
NPV and PV proportions. We used collected stand architectural 
data (e.g., trunk height, tree height, trunk radius, and crown 
radius) from the SPREAD campaign to parameterize FLIGHT. 
Their major characteristics are summarized in table 1. The 
simulated stands were horizontally distributed on a flat terrain 
according to a Poisson distribution with crowns of irregular 
conical shape and cylindrical trunks. Within the individual 
crows a spherical leaf angle distribution of the phytoelements 
was assumed. Additional parameters were fixed to model 
default or field measurement as described in Kôtz et al. (2004). 
Canopy reflectance was simulated as observed from nadir in 18 
spectral bands corresponding with specifications of CHRIS in 
Mode 3 (land). CHRIS aboard PROBA (Project for On-board 
Autonomy) satellite is an experimental smallsat that has the 
capability to provide combined hyperspectral and 
multidirectional sampling with high spatial resolution (~17 m) 
of selected terrestrial targets (Barnsley et al., 2004). In the 
‘land’ mode wavebands are selected specifically to monitor 
vegetation cover. The spatial size of a CHRIS pixel precisely 
matched to the FLIGHT scene dimensions (17 m). The solar 
angle was set as during CHRIS-PROBA overpass over Swiss 
National Park on 2004-06-27 (9 S : 24.0°, <f> s \ 162.8°, see Verrelst 
et al., 2008 for details). All spectral data were convolved to 
CHRIS using the CHRIS band centers and full-width-half- 
maxima (FWHM). 
To make sure that explicitly relationships between reflectance 
spectra and chlorophyll content are studied with no other 
influencing factors than canopy structure and woody elements, 
only the vegetation signal free of any contamination is needed. 
To achieve uncontaminated reflectance spectra, a solution is to 
apply a total correction for background signal by setting the 
background layer equivalent to a perfect absorber canopy 
background condition. In this case the optical characteristics 
and associated Cab range of the experimental overstory canopy 
signal was directly linked with biophysical parameters, without 
any atmosphere or background contamination. Once the 
initialization was done, one million rays penetrated in each 
experimental canopy. Finally a total of 7938 forest scene 
simulations (9 Cab x 14 crown LAI x 9 NPV-PV x 7CC) 
executed by PROFLIGHT provided the spectral sampling for 
the subsequent analysis of the contribution of woody elements 
and needle Cab content at stand level. 
3. RESULTS 
In FLIGHT, the radiant fluxes interacted with the woody 
elements and the chlorophyll-containing foliage which resulted 
into upscaled chlorophyll-induced spectral response. The 
averaged BRF ( BRF ) and chlorophyll-induced spectral 
dispersion (CV C ab) was once more calculated, now at stand level. 
Figure 2a and 2b shows the averaged BRF and associated CV Cab 
for the CHRIS bands and the SNP structural configurations 
(table 1). With a perfect absorbing background, only the 
scattering caused by the overstory canopy attributes that 
escapes into nadir direction are detected. This assures an