"A CI.
X. x4 pu p AP. T - rd "T A ci 4 'w Mw Sf qe AR A
4.3 Slope Aspect
À slope aspect can be expressed from DEM,
which is one of the most important items for
topographical analysis. In this study, the slope
aspect shows along the maximum inclination angle
at one target pixel. An index of slope aspect
accuracy is also used percentages of correct pixels.
In case of slope gradient accuracy, correct pixel
means difference with verification slope gradient
data indicates inside of 45 degree. Figure 9 shows
relationship between contour line interval and
correct percentage in each method. The correct
percentage shows very lower than elevation or
slope gradient, which is indicated less than 68%.
However, buffering method is almost located the
highest accuracy in all contour intervals.
= 70-00: er
i 0 o----Profile
65.00 [Fr > --4l]-- Window
| fg Buffering
Correct percentage (%
A C [91] eo
aso 55
e e e e
© o eo o
A
o
o
o
-
L
50 100 150 200. 250 30 3580 400 450
Contour line interval (m)
Figure 9 Relationship between contour line
interval and correct percentage of slope aspect
44 Stream Pattem
A runoff analysis or a stream pattern
generation is very popular application of DEM.
Usually, such analysis can be carried out by using a
grid series tank model. A precipitation is supplied
to each DEM grid that is one of the tanks. An inlet
content which means effective rainfall for
discharge is calculated by following equation.
Q,-KiR L? Qin: Inlet Content (n°)
Ki: Infiltration
R: Precipitation (m)
L: Grid Size (m)
The inlet content must discharge to next
grid according to slope aspect and velocity. That
is to say flow tracking. The slope aspect can be
calculated from DEM, the velocity can be
estimated from slope gradient which is also
calculated from DEM. And the flow in the grid
can be expressed by a continuous equation as
follows;
ps 3
Q EC d. At Q: Remaining Content (ni )
HAL q in 4 m Qin: Inlet (ms)
out: Outlet (m/s)
At: Time (s)
By using previous equations, stream pattern
can be drawn. In this study, a parameter of
infiltration was given 1.0, because purpose is just
DEM evaluation.
An index of stream pattern accuracy is also
used percentages of correct pixels. In this case,
correct pixel means difference with verification
discharge value indicates inside of 20 m?/s. Figure
10 shows relationship between contour line
interval and correct percentage in each method.
Buffering method is almost located high accuracy.
However, in 400m contour interval, buffering
method indicated the worst accuracy. The
discharge accuracy is influenced by slope aspect.
Though the slope aspect of buffering method kept
the highest accuracy, stream pattern accuracy
didn't keep it. This reason might come from
difference of aspect distribution.
65.00 ..
to fo - = profile
© -@---P
p --Ja-- Window
o *
S 60.00 | —f— Buffering
=
D
O
I
D
ssl
©
©
©
O 5000 i : à j | : :
100 150 200 250 300 350 400
Contour line interval (m)
Figure 10 Relationship between contour line interval
and correct percentage of stream pattern
4.5 Slope Stability
A slope stability analysis is also popular
application of DEM. Sometime we generate land
slide risk map or slope failure risk map from DEM.
The slope stability which means safety factor was
Figure 11-b Illustration of
Each Slice
5
Landslide
Elevation
C3 mi lee di m
ritical Circle
rs
" 1 M
Ie
WP» Grid Si
Length of Landslide nd ze
Figure 11-a Illustration of Fellenius Method
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996