Fatehi, Ahmad
5 Statistical analysis
An attempt was made to show whether any differences in NDVI values will come up when using methods like Box
Plot or an other statistical analysis. Figure 3 is an exploratory diagram of the NDVI values from 1989 to 1993 in the
study area. As figure 3 shows the NDVI values in 1989 generally lower than in the other years. The result of
meteorological study show that there was very little rainfall in 1989 (Jalali et al 1999). The NDVI values of 1989 are
very low as compared with the other years, therefore the NDVI values of 1991 are compared with other years except
on 1989.
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N= 39 39 39 39 39
NDVI89 NDVI90 NDVI91 NDVI92 NDVI93
Figure3: Box plots of average NDVI values of growing season in period 1989-1993. N is size of data, x-axis is year
and Y-axis is NDVI values. This figure shows that the maximum and also minimum NDVI value of 1991 is lower
than 1990, 1992, and 1993. The median of NDVI values in 1991 is also lower than in 1990, 1992 and 1993,
therefore greenness highly decreased in 1991. Moreover, to see weather there were any significant changes between
NDVI values of different years the t-test was applied on the NDVI value of 1990,1991,1992 and 1993. The results
are shown in table 1. The number of sample, mean, standard deviation, standard error, degree of freedom and t
values are shown in table 1. Result that is shown in this tables indicated decreasing of NDVI values in 1991 as
compare with 1990, 1992, and 1993 are significant with 95% confidence interval as shown in the last column of
table 1.
Table 1: Result of t-test on the NDVI
Paired Differences
95% Confidence
Interval of the
Std. Std. Error Difference Sig.
Mean Deviation Mean Lower Upper t df (2-tailed)
Pair 1 NDVI90 - NDVI9| 5.4149 3.2815 ‚5255 4.3511 6.4786 10.305 38 .000
Pair 2 NDVI91 - NDVI9i -2.5800 4.0753 6526 | -3.9011 | -1.2589 -3.954 38 .000
Pair 3 NDVI91 - NDVIOi -7.8010 2.9613 4742 | -8.7610 | -6.8411 | -16.451 38 .000
df = degree of freedom, sig. = related significant level to the t number. This table shows sig. Is less than 0.05
(considering 95 % confidence) therefore differences of NDVI values between 1991 and 1990, 1991 and 1992, and
1991 and 1993 are significant. This difference of NDVI values in 1991 is reduction in vegetation cover caused by
the pollution in 1991
6 Relationship between AVHRR (GAC) NDVI values and distance from the source of the pollution
Rainfall was similar in 1990 and 1991 and there was no significant change between these two years in temperature
therefore, 1990 was selected as reference year to do comparison on change of vegetation greenness (NDVI).
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B7. Amsterdam 2000. 429
