Geoscience Reference
In-Depth Information
Table 1.
Statistics of NDVI images.
Area
Date
Mean
Std.
Min.
Max.
1996/11/08
0.36
0.04
0.11
0.48
1999/03/06
0.32
0.04
0.13
0.43
1999/10/31
0.14
0.07
-0.22
0.33
A
2000/11/27
0.15
0.07
-0.14
0.35
2001/11/20
0.37
0.05
0.03
0.50
2003/12/17
0.15
0.06
-0.12
0.33
2004/11/19
0.35
0.06
0.05
0.54
1996/ 11/08
0.36
0.03
0.13
0.47
1999/03/06
0.36
0.04
0.14
0.48
1999/10/31
0.16
0.05
-0.20
0.38
B
2000/11/127
0.17
0.05
-0.09
0.33
2001/11/20
0.37
0.04
0.14
0.48
2003/12/17
0.20
0.06
-0.08
0.44
2004/11/19
0.39
0.05
0.10
0.57
Previous studies that quantifi ed the impact of large disturbances did not evaluate
the spatial structures of NDVI images in the study areas. To demonstrate the ability
of the variogram to depict landscape heterogeneity, spatial variability and patterns,
experimental variograms, and their variogram models were fi rst analyzed and fi t to
seven images of areas A and B (Figure 4 and Table 2). The models are obtained in two
processes such as parameter estimation (fi tting) and cross validation. Cross-validation
in Table 2 is a means for evaluating effective parameters for kriging interpolations.
In cross-validation analysis each measured point in a spatial domain is individually
removed from the domain and its value estimated via kriging as though it were never
there. In this way a graph can be constructed of the estimated versus actual values for
each sample location in the domain.
The three main features of a typical variogram model are (1) the range, (2) the
sill, and (3) the nugget effect. The sill is the upper limit that a variogram approaches
at a large distance, and is a measure of the variability of the investigated variable: a
higher sill corresponds to greater variability in the variable. The range of a variogram
model is the distance lag at which the variogram approaches the sill, and can reveal the
distance above which the variables become spatially independent. The nugget effect is
exhibited by the apparent non-zero value of the variogram at the origin, which may be
due to the small-scale variability of the investigated process and/or measured errors. In
this study, the variogram models of the seven NDVI images for areas A and B areas are
exponential models. The spatial variations (Sill; C
0
+ C) of NDVI images from high to
low are in 2003/12/17, 2004/11/19, 1999/10, 2000/11/27, 1999/03/06, 2001/11/20, and
1996/11/08 in area A. The spatial variations (Sill; C
0
+ C) of NDVI images from high
to low in area B are in 1999/10/31, 2000/11/27, 2004/11/19, 2003/12/17, 2001/11/20,
1999/03/06, and 1996/11/08. The spatial variations of NDVI images increase consid-
erably from 1996/11/08 to 1999/10/31 (after the Chi-Chi earthquake) in both areas
A and B. Similarly, small variations (Nugget effect) of NDVI images in 2003/12/17
(after typhoon Dujuan), 1999/10 (after the Chi-Chi earthquake) and 2000/11/27 in area
A are larger than those in 1999/03/06, 2001/11/20, 1996/11/08 and 2004/11/19. In area
