calculated by a ratio between driving moment and
resistance moment on the profile. When the
safety factor is calculated on every pixel, such risk
map can be generated. Fellenius method as slope
stability analysis was selected in this study. In
this method, landslide type is assumed rotational
slip (Figure 11). A landslide soil is divided into
some slices in order to calculate moment along the
critical circle. The driving moment(T) and
resistance moment(N) on each slice are calculated
by the following equation.
T zB.W. sino
N = R(C- L + tanp- W- cos a)
R Radius of Critical Surface (m)
€ Cohesion (t/m2)
0 Angle of Shearing Resistance (degree)
W Weight of Each Slice (t/m) (W =v, A)
Y Wet Unit Weight of Soil (t/m°)
A Area of Slice (m?)
o Angle between Horizontal Axis and the Base
of Slice (degree)
L Length of the Base of Slice (m)
Therefore safety factor(Fs) is calculated
by the following equation.
Fs iN
ST.
Originally, parameters of soil mechanics (C,
6, y) and radius of critical surface (R) should be
determined by experimental data and field survey
data on each pixel. In this study, those parameters
were given by constant value as follows:
R =200m, C = 2.0t/m?, ¢= 10°, y= 1.9t/m?3
When profile at target pixel was drawn along
the steepest direction, Other parameters can be
estimated by DEM. If this safety factor calculation
applied every pixel, slope stability risk can be
mapped.
An index of safety factor accuracy is also
used percentages of correct pixels. In this case,
correct pixel means difference with verification
safety factor value indicates inside of 0.2. Figure
12 shows relationship between contour line
interval and correct percentage in each method.
Buffering method is always located the highest
accuracy. The safety factor accuracy requires slope
gradient and slope aspect. Buffering method made
very good result for slope stability analysis.
6. CONCLUSIONS
When existing interpolation method used for
r — -$- - - Profile
8$ 4000| --J8-- Window
2 L fu Buffering
©
€ 30.00 L
D
2
5 5
Q
= 20.00 L
o
© § I
oO 10.00 1 1 1 1 L ey i
100 150 200 250 300 350 400
Contour line interval (m)
Figure 12 Relationship between contour line interval
and correct percentage of slope stability
small scale contour map, some problems were
occurred. For example, profile method has much
error in elevation value. Window method has
much error in slope gradient.
In this study, buffering method was
developed for continental DEM generation from a
small scale map. The developed method was
compared with existing methods on elevation,
slope gradient, slope aspect, stream pattern and
slope stability. In all items, the buffering method
showed the best results.
A contour line interval influenced accuracy of
DEM. When contour line interval becomes over
300m, a correct percentage becomes less than 50%. A
percentage of pixels which are consisted contour
line is about 10% in case of 300m contour interval.
Moreover, in case of 100m contour interval, the
percentage becomes 30%. Therefore, at least 20%
contour line information on whole map are required
for reliable DEM generation.
7. REFERENCES
[1] S. Viseshsin and S. Murai (1990), "Automated Height
Information Extraction from Existing Topographic
Map", International Archives of Photogrammetry
and Remote Sensing, Vol.28 Part 4, pp.338 - 346
[2] K. Fukue, Y. Kuroda, H. Shimoda and T. Sakata (1990),
" Simple DEM Generation Method from a Contour
Image", International Archives of Photogrammetry
and Remote Sensing, Vol.28 Part 4, pp.347 - 355
[3] F. Ackermann (1994), " Digital Elevation Models -
Techniques and Application, Quality Standards,
Development", International Archives of
Photogrammetry and Remote Sensing, Vol.30 Part 4,
pp-421 - 432
[4] G. Aumann and H. Ebner (1990), "Generation of High
Fidelity Digital Terrain Models from Contours",
International Archives of Photogrammetry and
Remote Sensing, Vol. 29 Part 4, pp.980 - 985
[5] M. Takagi, S. Murai and T. Akiyama, 1992,
"Generation of Land Disaster Risk Map from
LANDSAT TM and DTM Data", International
Archives of Photogrammetry and Remote Sensing,
Vol.29 Commission VII, pp.754-759
852
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
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