266 
% of variance between field of the index values 
Table 1. Vegetation .indices. 
VEGETATION INDEX 
. ACR0N0M0US 
1 
1 
BANDS AND 
CALCULATION 
l|2 1 
RATIO 
R 
1 
1 
IR/R 
1 1 
1 1 
1 SQUARE RATIO 
1 SOR 
1 
Vir/r 
1 § 1 
LOG RATIO 
Log R 
1 
1 
Log IR/R 
1 1 
NORMALIZED DIFFERENCES ND 
1 
1 
(IR-R)/(IR.R).0.5 
1 1 
iOuj I 
| TRANSF. VEGETATION 
| INDEX 
T.V.I. 
1 
1 
V(IR-R)/(IR+R)+0.5 
| GREENNESS 
1 
1 Greenness 
1 
1 
1 
1 
-aG-bR+cRIR+dIR 
l! 1 
PERPENDICULAR VEGET. 
INDEX 
1 P.V.I. 
1 
1 
1 
V(R -R ) 2 . (IR .1R ) 2 
S V S V 
lisi 
DIFFERENCES 
G 
1 
1 
IR - R 
11 1 
| DIFFERENCES-DIFFEREN 
| CES 
! 00 
(IR-RIR) 2 - (R-G) 2 
1 s 1 
1 s 1 
G ■ green; R « red; 
RIR - red and i 
frar 
ed; IR « near infrared 
% green vegetation cover plotted against calendar 
days . 
vegetation index-values ( ) and % of variance against 
calendar days ( ). 
Table II. Coefficient of correlation between vegeta 
tion index values and final grain yield versus diffe 
rent phenological stages. 
tation indices. 
maximum correlation between spectral vegetation 
values and final grain yield, belong to the 
perpendicular and normalized family indices. From 
flowering to the end of mulky-maturity phase, the 
maximum correlation is related to vegetation indices 
based on the difference concept. Before harvest, 
that is at the physiological maturity, all the 
indices considered in this study show the same 
correlation values. The results obtained point out 
that it is necessary to adopt different vegetation 
indices at specific phenological stages, if the 
maximum correlation values have to be exploited to 
forecast wheat yield by means of remote sensing 
data. This fact can be explained by observing 
Figures 1 and 2. In Fig. 1 three spectral profiles 
and the trend of the % of the soil vegetation cover 
are plotted against seven phenological stages. As 
can be observed, the ratio profile function (R) is 
more sensitive to the percentage of soil vegetation 
cover than the Greenness and Normalized functions 
(Greenness, ND). This fact is more evident in 
proximity of the maximum soil vegetation cover. That 
corresponds approximately to the heading phase. 
Since the soil vegetation cover is correlated with 
the leaf area index of the wheat (at least for soil 
vegetation cover 85%) and the LAI at the heading is 
strongly correlated with the final grain yield (if a 
normal climatic and environmental condition is