Monitoring Drought Using Coarse-Resolution Polar-Orbiting Satellite Data - Monitoring and Predicting Agricultural Drought: A Global Study - page 62

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46

[62],

Line
——
0.5
——
Norm
PgEn

Figure 5.3 Examples of spectral reflectance of soil and vegetation at different wavelengths.
Note the high spectral contrasts between green vegetation and soil in the red and near infrared
sp ectral regions (from Tucker and Sellers, 1986).

ve getation index (NDVI), and transformed vegetation index (TVI). The
R VI, proposed by Rouse et al. (1974) using Landsat multispectral scanner
(M SS) imagery, is a simple division of the reflectance values in the NIR
ba nd by those in the red band.

[62],

N ormalized Difference Vegetation Index The NDVI was also proposed
by Rouse et al. (1974) as a spectral VI that isolates green vegetation from its
ba ckground using Landsat MSS digital data. It is expressed as the difference
be tween the NIR and red (RED) bands normalized by their sum:

NIR − RED

NDVI = [5.1]

NIR + RED

The NDVI is the most commonly used VI because it has a desirable
measurement scale ranging from

−

1 to 1 with zero as an approximate
value of no vegetation. Negative values represent nonvegetated surfaces,
whereas values close to 1 have very dense vegetation. The NDVI and RVI
ha ve the ability to reduce external noise factors such as topographic effects
an d sun-angle variations.

Transformed Vegetation Index The TVI, proposed by Deering et al.
(1975), modifies the NDVI by taking its square root and adding a con-
stant of 0.50. This creates a VI scale consisting of mainly positive values
approximating a more normal distribution. There are no theoretical dif-
ferences between the NDVI and TVI in terms of quantitative values or