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# Full text

Title
Proceedings, XXth congress

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