The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008
1010
1995
119/31
05.15
7
2001
119/31
08.11
7
Table 1. Faku TM data
In addition, Faku county’s relief map of 1:50000 in 1975,
investigation report of land use in 1992, population statistic data
from 1995 to 2001 and other text data were collected.
3.2 Remote sensing data preprocessing
Data preprocessing mainly refers to do data correction, noise
removed, cut, and so on before the official use [2 l The chosen
data has no spots or various types of noise, So do data
preprocessing including putting the text data in database,
geometric correcting and radiometric correction for the remote
sensing images.
3.3 Determination of NDVI threshold
Vegetation index reflects the approximation relation between
the spectral response and vegetation cover. The essential
characteristic of desertification is the lower the productivity of
land. Changes in vegetation index can reflect the changing
process of land productivity. Therefore vegetation index can be
used as desertification monitoring indicators to monitor land
desertification and dynamic changes. NDVI is the abbreviation
for Normalized Difference Vegetation Index. It was confirmed
according to strong absorptions of chlorophyll in 0.69pm. Use
ratio between infrared and near infrared and linear combination
to express vegetation information. The Vegetation index can
reflect a series of physics quantity of the plants, such as index
of leafs area, plant overlays degree, amount of living creature
and plant category etc [3 ' 4] . The index of vegetation contain
many different expression methods, among them NDVI is
applied in the fixed amount research of plant overlays.
Calculation formula of NDVI is:
For the image of TM:
ND VI = (p4 - p3) !(pA + p3) I
p3 = TM images the three-band visible light
Determining the threshold of NDVI is the first step in extraction
of land desertification. Then we can use the threshold to
separate vegetation information and non-vegetation information
from land cover.
Through comparative analysis of the NDVI images and the
research area, select NDVI = 0.05 as the threshold, which can
separate vegetation information and non-vegetation information
from the research land cover, that is: if NDVI> 0.05, the area
was considered as vegetation; Instead, considered as non
vegetation information. The NDVI image of research area in
2001 is below (Figure 1).
Figure 1. NDVI image of research area in 2001
3.4 Desertification index system of research area
Through the sufficient district investigate and practice research,
divided the extent of desertification in the research area into
three degrees: slight desertification, middle desertification,
severe desertification. Specific quantitative indicators are
shown below (Table 2).
3.5 Result of the research area’s classification
Do remote sensing classification based on the index system in
the research area. The result is below(Figure 2.).
Where
p4
vegetation coverage
NDVI (Normalozed Difference
Vegetation Index)
PCA (3,2,1) synthesis features
of color images
slight desertification
<15%
<0. 14
shallow green, distributed in the
stope edge, the bands on both
sides of the river
middle desertification
<10%
<0. 10
shallow pink, distributed
around severe desertification
severe desertification
<5%
<0. 07
red or blue, distributed in the
waste dump, mining pits and
the surrounding settlements
Table 2. Extent of desertification hierarchical list in Faku county