ting up the first order polinomials by four anchor points 
of each case, and the further statistics have been 
performed. The following Table 1. is the result of statis- 
tics (unit is pixel). 
Table 1. 
  
| Indirect Approach | Direct Approach 
| 
  
| 
Average MSE | Mx=0.0150 My=0.0169 | Mx=0.0133 My-0. 0079 
  
| | 
Maximum Error | Ex=0.2486 Ey=0.6426 | Ex=0.1679 Ey=0. 4340 
| | 
  
Minimum Error | Ex=0.0011 Ey=0.0003 | Ex=0.0015 Ey=0.0002 
23 |. decl =. 
As there is one value of MSE of point for each 
case. So we have the average MSE of point. From Table |, 
we can find out that the error of point for direct 
approach is less than that for indirect approach. 
6.3 The precision of orthophotomap 
The final results of two approaches in rectifi- 
cation are two orthophotomaps, on which the check points 
have been selected to fit up the first order polinomials 
for examining the precision. Table 2 is the fitting error 
of point on those check points (unit is meter). 
  
  
  
  
  
  
  
  
  
  
  
Table 2. 
Point No. | Indirect Approach | Direct Approach | 
| line | pixel | line |. pixel | 
1 | 74.4 | 2.3 | 1.3 ! Tus 
| | m | | 
2 | 1.0 | 2.3 L. 5.27. 15: 04.4 | 
3 rs -10.8 | za. | 4.4 | 3.3 | 
4 -6.8 | -10.6 | 6.5 | 4.3 
5 | 1.9 | E | 5,5 | 74. 6 
6 us 8.9 | 73.9 | 29.8 | =f) 1 | 
1 | 26.5 | 6.1 = 3.8 | 
8 3 4. 6 | 14.4 | —8 2.3 | 
MSE 6.08 6.27 4.40 5.66 
  
Table 2 shows us that the rectification results 
have both the precision of subpixel.That is about one half 
of a pixel. Furthermore the precision of direct approach 
is higher than that of indirect approach. So taking into 
account the imaging geometry of SPOT image and employing 
the corresponding flexible algorithm for the rectifi- 
cation are very important to assure the quality of geome- 
tric processing. As the last results, Table 2 has 
proved the correctness, reasonableness, and flexibility 
of the principles and methods suggested above. 
7 Conclusion 
When performing digital geometric corrections for 
SPOT images, it is not only necessary, but also flexible 
to rectify the prejective distortion due to relief. The 
suggested principles and methods based upon the imaging 
geometry of SPOT have been successful in making first use 
of collinearity equations for linear-array scanned images. 
According to the characteristies of SPOT images, The 
employment of direct approach in rectifications has shown 
us for the first time that it has more advantages to this 
kind of images. 
34 
References, 
i. 
Wang Zhizhou, 1990. Principles of Photogrammetry 
€ With Remote Sensing ), Publishing House of Surveying 
and Mapping, pp. 1-10. 
Shu Ning, 1988. On the Digital Geometrical Recti- 
fications for the Spacelab Images by Using Collinea- 
rity Equations. Journal of Wuhan Technical University 
of Surveying and Mapping, Vol.13, No.3.