The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008
NDVI-NDVI mil
NDVI veg -NDVI soil
(8)
NDVI
Where, veg is the NDVI 0 f a p ure vegetation pixel
while I so ‘ l is the NDVI of a pure soil pixel.
NDVI
In theory, sod should be zero for most soil types, but it
changes from -0.1 to 0.2[11,12] because of the influences of
many factors. veg should be the maximum of ND VI,
but it will change with the spatial or temporal change because
NDVI
of the influences of vegetation types. Thus, veg and
NDVI
so ‘' can not be fixed values [13] even in the same image.
Vegetation type changes with the change of the land use type.
To the same land use type, vegetation types are same
NDVI
approximately [14], so the pixels’ veg are close to the
NDVI
same vegetation type; the pixels’ so ‘ l are close to the
same soil type too. Thus, the land use map and the soil map
NDVI NDVI
may be used to compute veg and so,i .
temperature can be seemed as island and be easily recognized.
Meanwhile, there was not a clear center of UHI in the center of
the urban. There structure of UHI was complex inlaying and
showed a pattern of distribution centers.
Table 1 represents the mean brightness of temperature of
Caidian, Hanna, Jiangxia, the center urban and the whole
region and the URI of the whole region for 14 years. It was
found that the mean brightness of temperature of the whole
region was increased from 13.37°C to 22.98°C. As the
development of urbanization, the quality of hot environment of
the center urban also degraded seriously. The mean brightness
temperature was increased in the study districts, especially in
the center urban. Comparison between the mean temperature of
Oct 26, 1988 and Oct 13, 2002 in the center urban shows that
the values increased 9.12 °C.
To analysis the change of UHI, Urban Heat Index (URI) was
introduced, which was based on the ratio of UHI area to urban
area. The greater the index is, the more intense of the UHI is.
The trend of UHI showed a obvious trend: as the urban area
expanded, the URI of Wuhan consistent increased, especially in
1991. The URI from 1988 to 2002 were 0.134, 0.153and 0.170,
respectively.
3.2 Characteristic of Vegetation Fraction and its variation
3. RESULTS AND DISCUSSION
3.1 Characteristic of LST and temperature variation
The brightness temperature of Wuhan from 1988 to 2002 could
be derived using formula (2) and (3). In order to minimize the
impact of the time difference, the temperature was normalized
to 0~1. Fig.2 shows the distribution of temperature
the mean brightness of temperature(°C)
1988
1991
1996
2002
Caidian
13.69
17.62
21.23
22.42
Hannan
13.19
17.31
21.33
23.16
Jiangxia
13.75
17.96
21.96
23.08
center urban
14.14
18.06
22.06
23.26
whole region
13.37
17.52
21.64
22.98
URI of
whole region
0.134
0.153
0.158
0.170
Table 1 the mean brightness of temperature of different district
and the URI of the whole region
This paper estimates vegetation fraction of Wuhan based on the
NDVI me thod for dimidiate pixel model. First, NDVI 0 f ¡ t
is computed; then, the land use map and the soil map of the
study area were used as reference to make a statistic of the
whole NDVI va i ue According to the frequency statistical
table, the NDVI frequency of 5% in the soil map was used
NDVI
110,1 , and that of 95% in the land use map was used as
as
NDVI NDVI NDVI
veg ; at last, sod and veg which obtained
above were taken into equation (9). So we acquired the
vegetation fraction map of the study area.
Figure.2 The normalized temperature distribution in
the study area onOct 26, 1988(a), Oct 23, 1991(b),
Oct 4, 1996(c) and Oct 13, 2002(d)
maps, from which we can see that the UHI had an increase
trend in the process of fluctuations for the past 14 years,
especially in the center urban. From 1996, the land surface
temperature of the urban, n which there is industrial areas, new
urban and densely populated residential areas, as much higher
than that of the suburb, and the area of high land surface
From the Fig 3, we can see that vegetation fraction in the center
urban, where performed as the center, was lower than that in
suburb. During the 14years, the vegetation fraction was
highest in 1988 and lowest in 2002.
Table 2 shows the change of vegetation fraction in the
multi-temporal. From 1988 to 1991, the average vegetation
fraction of the whole region has dropped, but the overall
downward trend is not very clear, only dropped 2.2%. The
vegetation fraction in the center urban decreased significantly,
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