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ORTHORECTIFICATION OF HR SATELLITE IMAGES WITH SPACE DERIVED DSM 
R. Cavallini“, F. Mancini*, M. Zanni* 
"DISTART, University of Bologna, V.le Risorgimento 2, 40136 Bologna, Italy 
roberto74@libero.it , fmancini@racine.ra.it , michela.zanni@mail.ing.unibo.it 
KEY WORDS: Remote Sensing, Orthorectification, EROS, DEM/DTM, High Resolution, Accuracy 
ABSTRACT: 
The paper addresses the item of orthorectification of high resolution satellite im 
achieved when Digital Surface Models (DSM) provided by other remotely sensed data 
processing, a DSM was obtained from an EROS (Earth Resources Observ 
ages and the assessment of the final accuracy 
are used. By using a photogrammetric 
ation System) high resolution stereo pair acquired over a 
portion of the city of Bologna (Italy). After the accuracy assessment of the terrain model trough the comparison with external Digital 
Elevation Model (DEM), a broad range of orthorectification procedures with high resolutior 
1 satellite images (Ikonos, QuickBird and 
EROS) have been therefore investigated by the authors. The accuracy in final positioning provided by the orthorectification of a 
QuickBird imagery with the EROS-derived elevation dataset was evaluated using a evenly spaced set of Ground Control Points from 
GPS survey. The achieved accuracy could meet the requirements needed in technical cartography specifications (to scale as large as 
1510000) updating of well recognizable features or entities and generic mapping procedures. 
1. INTRODUCTION 
In this paper the use of high resolution optical satellite data for 
both DSM generation and subsequently image orthorectification 
have been addressed in the area of the city of Bologna (Italy) 
and its surroundings. The EROS Al satellite, successfully 
launched on December 2000 by ImageSat International and 
operating in an asynchronous mode, on Feb. 24, 2002 recorded 
an along track stereo-pair over the study area under favourable 
weather conditions. Using the new photogrammetric processing 
utilities available in Geomatica v9.1 package and GCPs, a 2- 
pixel post spacing Digital Surface Model was generated from 
the stereo-pair. The DSM was therefore validated by means of 
an available terrain model obtained from digitizing the contour 
lines of 1:5000-scale topographic maps covering the area. After 
the validation, in order to achieve a procedure for geospatial 
data analysis entirely based on satellite data, an 
orthorectification procedure of a QuickBird image using the 
EROS DSM was tested. The final result is represented by a 
satellite map that could be useful for updating the existing 
cartography (see published papers from the authors reported in 
the references section). 
2. MATERIALS AND METHODS 
Hereafter the EROS stereo-pair for DSM generation, the high 
resolution QuickBird satellite data and ground GPS survey 
methodologies will be briefly discussed after a description of 
the study area. 
2.1 Site location 
The area surrounding the city of Bologna is characterized by a 
predominantly flat topography (northern area) and a slightly 
hilly portion (southern area) with peaks that do not exceed 400 
m above sea level. The most built-up area is almost entirely 
extended in the flat portion. Figure 1 shows a perspective view 
of the area: the high resolution multispectral QuickBird image 
has been draped over the terrain model. 
  
Figure 1. Site location and main topographic features of the 
investigated area 
2.2 The EROS stereo images 
The EROS images were purchased in the 1A raw format. They 
have been initially corrected for sensor geometrical distorsion 
using the camera model, provided by the vendor, into the 1B 
format with its characteristic trapezoidal shape. After the 
resampling the GSD (Ground Sample Distance) of the 1A 
images, equal to 2.4 m, is transformed into an average pixel 
spacing (2.268 m 2.290 m respectively). 
In Table 1 some technical specification of the EROS 1A stereo- 
pair are summarized. 
  
  
  
  
  
  
  
Image name ITA1-e1123941 | ITAI-e1123943 
Date of acquisition Feb. 24, 2003 Feb. 24, 2003 
(hour) (9:34:16) (9:35:36) 
Along-track angle 30.2? 30.6? 
Across-track angle 8.0? 198.0? 
Scene size (km) 12.9 x 17.9 km 12.8x 17.9 km 
; ; ; 7560 x 5980 7640 x 5900 
Scene size (pixel) : ui 
pixel pixel 
GSD 2.4m 2.4m 
  
  
  
  
  
Table 1. Basic properties of the EROS 1A format stereo pair 
used in the DSM generation 
In figure 2a and 2b the stereo images in the 1B version are 
shown. 
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