THE INFLUENCE OF TERRAIN TYPE AND DENSITY 
OF DIGITAL ELEVATION MODEL ON THE GEOMETRIC 
QUALITY OF SATELLITE ORTHOIMAGES 
Dr. Farrag Ali FARRAG 
Lecturer in the Civil Engineering Dept. 
Faculty of Engineering, Assiut University 
Assiut- EGYPT 
Commission IV, Working Group 4 
KEY WORDS: Geometric, Quality, SPOT, Orthoimage, DEM/DTM 
ABSTRACT 
Orthoimages generated from SPOT satellite images can contribute significantly to image map production and to GIS 
applications. The quality of the extracted data from orthoimages depends mainly on the accuracy of the input data and the 
used mathematical modals. 
The aim of this paper is to study some factors influencing the geometric accuracy of SPOT orthoimages. The emphasis is on the 
effect of terrain type and Digital Elevation Model (DEM) density (grid size), on the geometric accuracy of orthoimages applying 
anchorpoints and pixel by pixel techniques. 
Real data is used in this study, where a SPOT-1 image with mirror looking angle equal to 25 degrees and the corresponding 
DEM data were used. The original DEM data has a regular grid of 100 by 100 m covering about 140 square kilometers of 
flat, moderate and rocky mountains areas. The study area is divided into smaller areas according to terrain roughness. In the 
first stage the orthoimage was generated using the original DEM data. In the second stage orthoimages were resampled using a 
courser DEM (grid size varying from 200 by 200 m. to 1000 by 1000 m.), where some points were omitted from the original DEM 
data. The orthoimage generated in the first stage is considered as a reference, in order to examine the orthoimages obtained in the 
second stage, and the relative geometric accuracy is demonstrated for different terrain types with different grid sizes. Computer 
programs were written by the author in order to carry out the resampling algorithms of this study. 
Although the results of this study discuss the relative geometric accuracy of orthoimages, these results declared the influence of 
terrain type and DEM grid size on the accuracy of the end product ofan orthoimage system. In addition the results of this 
study give detailed comparison between the anchorpoints (which has some computational advantages) and the pixel by pixel 
techniques. This comparison is demonstrated for different terrain types and different DEM densities . 
1. ORIENTATION ALGORITHM 
: ; ; along the normal to the image and pointing away from the 
A rigorous photogrammetric approach is applied to SPOT earth, The (+x) is the direction of satellite flight. The linear 
level 1A image in order to determine its time dependent ; _ aL 
: : array itself passes through the principal point in the focal 
orientation parameters. The working coordinate system, : i : 
: : ; plane where the x coordinate is zero. The x coordinate of 
image coordinate system; and the mathematical model used an à ; ; : : 
for ortentine SPOT images will’ be briefly desertbed the (6000 line * 6000 pixel) image is the line number of this 
OF onenime Images ind pixel (-3000 < x <+3000) and it is used as a measure of 
time at which a point was imaged. The (+y) is the direction 
1.1 Coordinate Systems Used in the Algorithm that results in a right hand coordinate system 
A SPOT scene covers a large area of the earth's surface. To 
avoid distortions caused by earth curvature and the map 1.2 Mathematical Model 
projection characteristics, the Geocentric Coordinate The principle of an extended bundle solution is applied to the 
System (X Y Z )isused as the working coordinate system. ground control points and their corresponding points in the 
Coordinates of ground control points given in geographical SPOT image. The photogrammetric standard bundle solution 
coordinate system (latitude, longitude and height) or is based on the following space resection formula: 
Universal Transverse Mercator (UTM) coordinates will be 
transformed to the geocentric system. : | x | |X-XO| 
The image coordinate system (x, y and z) is constructed with ly | =s* Mx) * Mg) * Mo) * | Y - YO| (1) 
the positive z direction at the center of a particular scan-line, 1c | IZ-ZO| 
246 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996 
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