The International Archives of the Photogramme try, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008 
1384 
(DEM). In this step we used 50 x 50 meters resolution DEM 
due to it is available freely. In order that, 3D coordinate 
calculation was done using triplet images of ALOS PRISM. It 
is possible to apply the calculation to other high resolution 
optical sensors mounting with pushbroom scanners such as 
SPOT 5 and Quickbird since ALOS PRISM sensor had same 
function with them. At last, the generated DSM is evaluated by 
validation points. In the registration process, 14 ground control 
points and 8 validation points were introduced; resulting 
X=0.97, Y=0.99 and Z=1.85 pixels in accuracy. This result was 
produced by stereo matching between nadir and backward 
looks. Even though the result measurement is bias with some 
errors, this result could be improved by triplet image matching 
and adding more validation points in future. 
As of main a requirement, the study is investigated the potential 
of usage of ALOS-PRISM DSM in slope failure disaster 
monitoring with the area consistency of 10 square meters. As 
detectable region of interest, it is possible to detect greater than 
5 square meters of the area because the spatial resolution of 
input image is 2.5 meters according to spatial resolution of 
ALOS PRISM sensor. Finally, changed detection result is 
evaluated. In the evaluation process, we created evaluation 
grids. Those grids are used to measure statistic values of the 
result image. Recently, this research work is applying to the 
land displacement monitoring project to prevent risky landslide 
disaster in the mountainous area. Future, we could be applied 
triangulated irregular network (TIN) model to time series DSM 
data sets together with evaluation grid statistic measurement. 
2. SUDY AREA & DATA INFORMATION 
2.1 Study Area 
Study area is located on 4 th isle of the Japan. Geographically, it 
is a small portion of Kochi Prefecture of Shikoku Island and it 
had uneven elevated area. The area is one of the most landslide 
risk areas of Japan by proving many geological faults. Many 
landslides are happened in this area. 
2.2 Data 
There are two datasets are used in this study; a low resolution 
DSM data and ALOS-PRISM triplet data. The PRISM data is 
used to generate new DSM to measure land-form changes and 
land displacement. Some characteristic of ALOS-PRISM data is 
listed in the table 1. 
Orbit 
Sun-Synchronous Sub-Recurrent 
Repeat Cycle: 46 days 
Sub Cycle: 2 days 
Altitude: 691.65 km (at Equator) 
Inclination: 98.16° 
Attitude 
Determination 
Accuracy 
2.0 x 10‘ 4 degree (with GCP) 
Position 
Determination 
Accuracy 
lm (off-line) 
Data Rate 
240Mbps (via Data Relay Technology 
Satellite) 
120Mbps (Direct Transmission) 
Number of Bands 
1 (Panchromatic) 
Wavelength 
0.52 to 0.77 micrometers 
Number of Optics 
3 (Nadir; Forward; Backward) 
Base-to-Height ratio 
1.0 (between Forward & Backward) 
Spatial Resolution 
2.5m (at Nadir) 
Swath Width 
70km (Nadir only) / 35km (Triplet 
mode) 
S/N 
>70 
MTF 
>0.2 
Number of Detectors 
28000 / band (Swath Width 70km) 
14000 / band (Swath Width 35km) 
* PRISM cannot observe areas beyond 82 degrees south and north 
latitude. 
Table 1: Characteristic of ALOS-PRISM data 
PRISM sensor is equipped with three optical independent 
systems to acquire digital surface model (DSM) with high 
spatial resolution. Three temperature stabilized radiometers of 
optical systems are pointing to forward, nadir and backward 
views. Thus, the sensor is set to generate 5 meters elevation 
data which is corresponded to the 1:25,000 scales in 
topographic map together with 2.5 meters spatial resolution. To 
maintain base-to-height ratio to 1, forward and backward 
radiometers are inclined ±23.8 degrees outwards from the nadir. 
To adjust the Earth rotation effect, each radiometer could use 
electrical pointing within ±1.2 (+1.5) degrees in cross-track. 
Thus, 35 km wide triplet images are acquired without yaw 
steering the satellite. Lossy compression with JPEG format is 
used to compress acquired data when it transmitted to ground 
station (Ohta, 2006). Pointing angle +1.5 degrees in cross-track 
is confirmed by PRISM first image observation (Tadono, 2006). 
The scanning geometry of PRISM sensor is shown in figure (1). 
46 seconds 46 seconds 
Figure 1: Scan geometry and triplet image of PRISM sensor 
3. TRANSFORMATION MODELS 
As the prior need of DSM generation, the selected stereo 
images pair must be ortho-rectified and corresponding pixels 
location should be geometrically corrected. Generally, RPC 
model is applied to stereo pair to generate ortho-rectified 
images and to correct pixels correspondence (Dowman, 2000; 
Tao, 2001). Consider producing high accurate ortho-images, 
remaining errors cannot adjust by the sensor alignment and 
GCPs are necessary for precise geometric correction (Fraser, 
2005; Hashimoto, 2006). To produce very precise geometric 
corrected one, it is need to collect very accurate ground control 
points by GPS-VRS. The standard accuracy of GPS-VRS could 
be acquired below the centimetre level.