The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008 
442 
First the histogram of master image is considered as reference 
and the other histograms are matched to it. Then the orthophoto 
of all images are produced with a regularly spaced grid DSM 
and using the backward projection approach. As mentioned 
before, during the ortho-rectification, the occluded areas are 
replaced by the grey values of the features and double mapping 
occurs. Figure 8 illustrates this problem. As shown in this 
Figure, in close range case the occluded areas are comparatively 
extensive. 
Figure 8: true orthophoto with double mapping due to the use of 
DSM. 
The occluded areas of the orthophoto are detected by using the 
height-based raytracing method. This process is performed for 
all images. In this method the optimal search interval should be 
determined based on the orthophoto pixel size so that the 
visibility of all pixels analyzed. The output of this process will 
be orthophotos with occluded areas appearing in black color. 
Figure 9 indicates the detected occluded areas for the test object. 
To obtain the final true orthophoto the mosaic procedure must 
be done. The final true orthophoto of the cubic object is shown 
in figure 10. 
□ 
Figure 9: true orthophoto with occluded areas marked black. 
Figure 10: Final true orthophoto of the test cubic object. 
5 CONCLUSION 
Digital true orthophoto generation in urban areas has most 
importance. The basic challenges are the occluded areas and 
double mapping. Therefore, the true orthophoto is generated 
from four close range images of a cubic object. In close range 
case the distance between camera and object is small and as a 
result the occluded areas are extensive. So an efficient method 
must be considered for occluded areas detection. The height- 
based raytracing method is applied that is not time consuming 
and the result of that has high quality compared to the other 
methods. The histogram matching and mosaic procedure is done 
to restore the occluded areas. Experimental results indicate that 
a smoothing process is still need to make the seamlines 
completely invisible. 
REFERENCES 
Amhar, F., Josef, J., C. Ries (1998). The Generation of True 
Orthophotos Using a 3D Building Model in Conjunction With a 
Conventional DTM, International Archives of Photogrammetry 
and Remote Sensing, vol.32, Part 4, pp. 16-22. 
Bang, Ki-In., Habib, A., Kim, C., Shin, S., (2007), 
Comprehensive Analysis of Alternative Methodologies for True 
Orthophoto Generation From High Resolution Satellite and 
Aerial Imagery, American Society for Photogrammetry and 
Remote Sensing, Annual Conference, Tampa, Florida, USA, 
May 7-11. 
Braun, J., (2003). Aspects on True-Orthophoto Production. 
Proceedings, 49th Photogrammetric Week, pp. 205-214. 
Stuttgart, Germany. 
Ettarid, M., Ait M’Hand, A. and Aloui, R (2005), “Digital True 
Orthophotos Generation”, Proceedings of the FIG Working 
Week, Cairo, Egypt, April 16-21,11 p. 
Schickler, W., Thorpe, A., (1998). Operational Procedure for 
Automatic True Orthophoto Generation, International Archives 
of Photogrammetry and Remote Sensing, vol. 32, Part 4, pp. 
527-532. 
Yamada, K., Odaa M., (2004). Discussion on the Generation 
Method of Simplified True Ortho at the Urban Area Using The 
Area Sensor Type Digital Aerial Camera “DMC”, International 
Archives of Photogrammetry and Remote Sensing, vol.XXXV- 
B3, pp. 942-947.