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1 Wiley 
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AUTOMATIC RELATIVE REGISTRATION 
OF SPOTS IMAGERY FOR COLOR MERGING 
Leong Keong KWOH and Xiaojing HUANG 
Centre for Remote Imaging, Sensing and Processing, National University of Singapore, 
SOCI, Level 2, Lower Kent Ridge Road, Singapore 119260 
crshxj@nus.edu.sg 
KEY WORDS: Multispectral, Registration, Correlation, Multiresolution, Orthorectification, SPOT, Sensor Model 
ABSTRACT: 
An automatic process for relative registering and merging the panchromatic and multispectral images of SPOTS is presented. The 
automated hierarchical local registration process makes use of feature information to select tie points from both images. The tie 
points are then used to refine the sensor model of the multispectral image by method of least squares solution. Both panchromatic 
and multispectral images are then orthorectified to a geo-referenced coordinate system using the available one kilometre gridded 
Digital Elevation Models (GLOBE or SRTM DEM). The merged high resolution colour imagery is then obtained by multiplying 
each spectral bands with a sharpening factor computed from the intensity values of the orthorectified panchromatic image and the 
corresponding pixels of the orthorectified multispectral images. 
I. INTRODUCTION 
The SPOTS satellite carries two high resolution geometric 
instruments (HRG) enabling it to map large area of the Earth 
(60km x 60km). Each HRG instrument on-board the SPOTS 
satellite images the ground with the panchromatic (PAN) band 
(2.5 m or 5 m resolution) pointing slightly forward (0.529?) and 
the multispectral (XS) bands (10 m resolution) slightly 
backward (-0.529?). This results in a slight relief displacement 
in opposite directions for the panchromatic and multispectral 
images and also a time interval of about 1.5 seconds between 
the PAN and XS images over a same point on the ground. 
Therefore there is a need to co-register both images together 
and to eliminate the relief displacement before they can be 
merged into a high resolution colour image. 
Christophe Latry and Bernard Rouge (Christophe Latry, 2003) 
used physical sensor models plus massive local image 
correlation to register the images and eliminate the relief 
displacement. This technique, however, may not be effective if 
the images have high amount of cloud cover or water bodies. 
Kwoh, in an earlier work (Kwoh, 2003), used a method based 
on coarse orthorectified PAN and XS images with the publicly 
available Ikm gridded GLOBE DEM to eliminate the relief 
displacement differences due to the different look direction in 
the Pan and XS image; and then using the simple affine 
transformation to register the two orthorectified images. The 
method had assumed that both the Pan and XS image have no 
relative orientation bias differences, which is logical since the 
Pan and XS images were taken at a mere 1.5 second apart. 
In this paper, we present an improved method where the PAN 
and XS images are registered relative to one another by refining 
the physical sensor model of one image relative to the other 
(instead of the simple affine transformation). The orthorectified 
images generated with the same DEM will then be well 
registered to one another and will have no relative relief 
displacement errors. Any orientation bias differences between 
the PAN and XS image will also be removed during the sensor 
model refinement process. 
The method is implemented in 4 steps as shown in Figure 1— 
(1) Automated image matching and tie point selection; (2) 
Relative SPOTS sensor model affinement for XS image with 
respect to PAN image with tie points; (3) Orthorectifying both 
PAN and XS images to a common georeferenced coordinate 
system by using one kilometre resolution GLOBE or SRTM 
Digital Elevation Models (DEM); (4) obtaining high resolution 
colour image by merging the high resolution (2.5m or 5m) 
panchromatic image with the lower resolution (10m) 
multispectral image. 
XS image — 14 
(10 m/pixel) 
    
  
PAN image — 1A 
(2.5 m/pixel) 
     
  
—línage Resample= — Corner Detection ~~ -— age Resample: > 
( | PANimage | [ Comers. [ . XSimage - 
|. (Muttiresolution) | — tae |. (Mutti-resolution) 
—]linage Matching: > 
Ii XS Sensor Model 
(Tie Points/ | . .—XS Sensor Model — . 
GroundPoints| ^ ~~. Refinement .— 
PAN Sensor Model ( XSRefined | 
  
(Sensor Model) 
Ls Orthorectify SRTM DEM —Drthorectify -—-.—3À 
'XS Ortho image) 
v, (0 mipixel) 
  
  
PAN Ortho Image] 
(25 mipixel) | 
<<Color Merginÿ — 
{Merged multispectral Image) 
Ë {2.5 mipixel) | 
Figure 1. Relative registration and colour merging diagram