International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B-YF. Istanbul 2004 
  
  
  
  
  
  
  
Figure 1. The study-area of Thessaloniki, Greece 
3. ORTHORECTIFICATION 
As it is mentioned above, through the process of 
orthorectification the two Spot images, the panchromatic and 
the multispectral, are transformed into an orthogonal projection, 
which makes them as accurate as a map in the same scale. This 
is accomplished through the following procedures. 
3.1 Geometric model and projection 
The accurate geometric model, taking into account the 
geometry of the acquisition and by recovering the distortions 
that exist describes the relation between the sensor and one 
ground reference system. In classic Photogrammetry this 
relation between camera, image and ground is described by the 
collinearity equations. This cannot be applied in remote sensing 
because the elements of the exterior orientation are not stable. 
This is due to the great speed of the satellite and the long time 
of acquisition. 
In the case of the spot geometric model, it is considered that 
the satellite has steady movement during the time of the 9 
seconds (duration of the acquisition of one spot image) and one 
scanning line is defined as a reference line. This can be the first 
or the central line. In this way, and as the exterior orientation of 
the reference line has been defined; the exterior orientation of 
all the rest lines is also determined, based on the distance 
between them, the changes of the position of the project center 
and the rotation angles. 
The mathematical spot model is a broaden model of collinearity 
equations with a difference that the satellite's route is steady 
during the acquisition time so its movement can be modelled 
with a polynomial of 1* and 2" grade. The exterior orientation 
in a polynomial of 1* grade includes 6 parameters that describes 
the route (position and speed), 
X (=X, + X't (1) 
YyX,- Yt (2) 
Z(09Z,* Zt (3) 
where — Xo, Yo, Zo * coordinates of projection center of 
reference line 
X’, Y’, Z’ = velocities in each direction 
and 6 parameters that describe the satellite orientation, 
ot)» o, *o't (4) 
140 
e(t)» Qo +t (5) 
K(t)- K, -k't (6) 
where  ®, Q, K,= 3 rotation angles 
Q0, Q9, K'7 their change rates 
The result is a model that has 12 unknown parameters and the 
time as an independent variable for the 6000 lines of a spot 
image. 
The Erdas IMAGINE has the option of choosing this geometric 
model during the process of the orthorectification. 
As far as the projection is concerned, the EGSA'87 is chosen, 
because this one is the official projection of Greece. 
3.2 DTM (Digital Terrain Model) 
The integration of DTM into the two images is the most crucial 
part of the orthorectification process, as its role is to eliminate 
the relief displacement. Moreover the DTM quality affects the 
accuracy of planimetry in orthophotos. The DTM that is used is 
a mosaic of two different DTMs in order to succeed the utmost 
accuracy. Both of them were produced with photogrammetric 
proceedings from aerial photos of scale 1:10000 and 
1:20000.The grid size is 25 meters. 
3.3 Control points 
The role of the ground control points (GCPs) is to define the 
exterior orientation elements of one image. Their accuracy is of 
a great importance because affects the accuracy of the 
orthoimage. Except for their high accuracy, it is 
recommendable that they have good geometry. This means that 
they should have symmetric position over the image and cover 
the whole area of interest. 
At this point it should be mentioned that the ground control 
points in this project were obtained from GPS measurements 
and from other existing orthoimages that depict the same area. 
For this reason they have different accuracy. 
For the rectification of the panchromatic image, 57 control and 
15 check points were used, and for the multispectral 61 control 
and 15 check points. 
3.4 Resampling 
Most geometric transformations lead to pixels that do not 
coincide with the original image. Resampling is the process of 
calculating the intensity of the new pixels with one method of 
interpolation. The data values for the pixels are interpolated on 
the new grid from the values of the source pixels, an invaluable 
procedure in the generation of an orthoimage. 
In Erdas IMAGINE the following methods of interpolation are 
supported: 
|. Nearest neighbour: uses the value of the closest 
pixel to assign to the output pixel value 
2. Bilinear interpolation: uses the data file values of 
four pixels in 2 x 2 window to calculate an output value 
with a bilinear function 
3. Bicubic interpolation: uses the data file values of 
sixteen pixels in 4 x 4 window to calculate an output value 
with a bicubic function