804 
in Los Angeles. The photos are arranged so that the terrain can be 
recognized by stereo scopy. Fig. 4 shows the distribution of geo 
metric distortion of the uncorrected image. 
5. DTM Generation from SPOT Stereo Pairs 
For generating DTM from SPOT stereo pairs, it is necessary 
to determine precisely the position and attitude of sensor and to 
recognize conjugate points. The former is the problem of exterior 
orientation and the latter is that of stereo matching. 
Exterior orientation can be made by the same method described 
in chapter 2 and 3. Since SPOT HRV imagery is sensed with a linear 
array sensor, the equation (1) is modified into (8). 
x = 0 (8) 
y=(I-(N + l)/2)*d 
where d : interval of array element 
Other notations are the same as chapter 2. The position and the 
attitude of sensor can be expressed by monomials and by trinomials 
of line number J, respectively. 
Although it is necessary for precise orientation to get the 
sufficient number of GCPs from the whole area of imagery, plenty of 
remarkable points for GCPs may not exist in some areas. The algo 
rithm developed by the authors can utilize feature lines like roads, 
rivers or shorelines and the data of geodetic observations like 
slant distances or levels as control data. 
The methods of conventional stereo matching based on image 
correlation have the problem of accuracy and reliability. In order 
to improve this weakness, our algorithm adopts a triplet matching 
method. A conjugate point is searched under the constraint that 
three bundles should Intersect at a point as shown Fig. 5. 
The result of DTM generation is introduced. The test area is 
located on the mountainous area near Mt. Fuji. Fig. 6 shows the 
date and off-nadir angles of SPOT imagery. Fig. 7 and Fig. 8 shows 
contour image of actual DTM and that of measured DTM, respectively. 
The contour intervals of both images are 40m. The accuracy is the 
RMSE of 21m and the maximum error of 68m, which is more accurate 
and reiiabLe than that by a conventional method. 
6. Concluding Remarks 
The summaries of the studies introduced in this paper are as 
foilows. The image distortions caused by off-nadir viewing can be 
corrected by the method based on photograinmetric principles. The 
distortions of SAR imagery can be corrected using DTMs. The DTM 
generation from SPOT stereo pairs are more accurate and reliable by 
the triplet matching method.