1073 
As far as the azimut angle is concerned we calculated a standard deviation a(cp) of 2.5°. Equation of 
regression constrained to the origin was : 
^estimated - ^ ^measured ~ 0-99, n - 130) 
3.2. Reconstruction of the maize cover 
A perspective new of the whole three-dimensional model of the maize cover is presented by figure 5. 
Comparison between estimates and real (X. Y. Z) control positions shows that no significant bias was observed 
in the reconstruction, but the standard deviation of the Z estimates was greater than that of the X and Y 
coordinates. Owing to the small B / H ratio (figure 2). the two camera optical axes were almost parallel to the Z 
axis, and therefore perpendicular to the X-Y plane, and that was at the origin of the difference between the 
precision on Z and X. Y estimates. 
Figure 5 : 3-D representation of the 
reconstructed vegetation cover: side 
view projections. The darker grey level 
of the vegetation elements means 
thecloser position to the observer. 
3.3. Estimation of structural parameters : leaf orientation and area density 
The triangulated 3-D leaf structure gave the possibility to study the leaf zenith and the leaf azimuth 
distributions. Another parameter of interest for the vegetation studies is the midrib azimuth angle estimated as 
the azimuth angle of the line segment betw een the tip and the insertion point of the leaf. 
The horizontal profile of the LAI (Leaf Area Index) and the vertical profile of the LAD (Leaf Area 
Density) were es tima ted (stems not considered). The obtained LAD STER£0 (Z) was compared to the 
LAD PLANTPR0FIL£S (Z) resulting from the "Plant Profiles Method" which has been performed during the same 
experimental period. The "Plant Profiles Method" considers that maize displays its foliage in a vertical plane 
and requires photographs of 20 plants representative for the population. The length and width of all present 
leaves were measured and their midribs were digitized from the pictures. Using these data and the digitized 
plant profiles, the LAD PLANTPR0F,LES (Z) and its standard deviation for the 20 plants are computed. 
2CX405C 60 70 8C90 
zenith angle (degrees) 
Figure 6 : Estimation of the zenith angle : 
- by the Plant Profiles Method (dotted line ) 
with the confidence interval (dashed line ): 
- by stereovision (solid line ).