1000 
  
  
  
  
  
  
  
  
  
3.5 ç 
3 | * Series 1 ; J 
- ef Series 2 cos 
s ia ee oe 
  
  
  
  
  
  
  
| l ] | 
  
200 300 400 500 
Grid 
RMS(of Vec. Disp.){Pixels} 
Figure 10 Comparison between Flat, Moderate and Rough Terrains. 
Series: 1=Flat Terrain 2= Moderate Terrain 3= Rough Terrain 
600 700 800 900 1000 
Size {m} 
  
  
  
6. CONCLUSION 
The influence of density of Digital Elevation Model (DEM) 
on the geometric quality of SPOT  orthoimages is 
demonstrated. This influence is evaluated for different terrain 
types and using a high oblique SPOT image with 25 degrees 
mirror locking angle. Two different techniques for 
orthoimage production were examined, anchorpoints and 
pixel by pixel techniques. The demonstration of the influence 
of these factors was based on comparing the orthoimages 
produced using course DEMs with the orthoimage of the 
same area but produced using dense DEM. 
i- As a result of examining orthoimages obtained by each of 
anchorpoints and point by point techniques, one can 
conclude the following: 
Concerning anchorpoints technique: 
. For all terrain types, the eight parameters transformation 
within the DEM grid give more accurate results compared 
with affine transformation. 
Concerning pixel by pixel technique: 
. For flat terrain, the best results is obtained by interpolating 
height within the grid by inverse distance method. 
. For moderate and rough terrain, the bilinear interpolation 
method give the best results. 
. Nearest neighbour method is not recommended for all 
terrain types because it gives the worst results. 
li- As a result of comparing orthoimages obtained by the 
anchorpoints and point by point techniques, one can 
conclude that: 
- For flat terrain, pixel by pixel technique gives better results 
at all grid sizes. 
. For moderate and rough terrain, although it was expected 
that the pixel by pixel technique will give better results, the 
two techniques give almost the same results. For such terrain 
types it is recommended to incorporate and test more 
elaborate algorithms for height interpolation such as 
patchwise or global methods. 
iii- Concerning the influence of terrain type: 
For all terrain types, the accuracy of orthoimages are 
decreased almost linearly by decreasing the density of the 
DEM. The orthoimages of flat terrain are less affected by the 
decrease of DEM density compared with moderate and rough 
terrain; and the orthoimages of rough terrain are most 
affected by the decrease of DEM density. 
7. REFERENCES 
Balce, A. E. (1987) Determination of Optimum Sampling 
Interval in Grid Digital Elevation Models (DEM) 
Data Acquisition. Photogrammetric Engineering & 
Remote Sensing Vol. 53, No. 3, pp. 323-33. 
Farrag, A. F. (1991) Map Updating Using Digital Techniques 
with Emphasis on SPOT Images. Ph. D. Dissertation 
Assiut University Faculty of Engineering. 
Gugan, D. J. (1987) Topographic Mapping from SPOT 
Imagery. Photogrammetric Record Vol. 12 No. 9, 
April, pp. 349 - 355. 
Michele, C and Michel C. (1981) The SPOT Satellite 
Remote Sensing Mission. Photogrammetric 
Engineering & Remote Sensing Vol. 74, No. 8, 
pp.1163-1171. 
251 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996