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This paper presents an investigation into the operational comparison of SPOT triangulation to build GCP library by
analytical plotter and DPW(digital photogrammetric workstation). GCP database derived from SPOT image can be used
to other image sensors of satellite, if any reasons, such as lack of topographic maps or GCPs. But, general formulation
of a photogrammetric process for GCP measurement has to take care of the scene interpretation problem. There are two
classical methods depending on whether an analytical or DPW is being used.
To conduct the operational comparisons, five models of SPOT panchromatic images(Level 1A) and negative
films(Level 1AP) were used. Ten images and film products are used among the three viewing angles to get the best
stereoscopic image quality. Photogrammetric measurements were carried out in a manual mode on the P2 analytical
plotter and the LH Systems DPW770.
2 MEASUREMENT OF GROUND CONTROL POINT-METHODOLOGY
2.1 The SPOT Geometric & Functional Model
SPOT scenes are segmented and delivered in an average GSD of 10m for the panchromatic image or 20m for the
multispectral mode in 60kmx . 60km of ground coverage. For photogrammetric processing, the required SPOT product
is processed to level 1A; only radiometric correction has been done. Level 1AP refers to hard copy negatives with
specially marked image corners for fast and more accurate inner orientation measurement
The link between a ground control point and its image coordinate can be represented as follows, (Azubuike & Alfons,
1996).
Xg = Xs + S Ri Rb Rs xP
where
e xP = image coordinates vector.
e Xg = ground coordinates vector in the earth centered inertial geocentric coordinate system
e Xs = satellite position vector in the earth centered inertial geocentric coordinate system
e S = scale factor
e Ri = Rotation from the orbital reference system to the earth centered inertial geocentric coordinate system
e Rb = Rotation between the attitude reference system and the orbital system.
e Rs = Rotation between the sensor and the attitude reference system
2.2 Methodology
GCPs play a very important role in image rectification. The number and distribution of the GCPs and the accuracy of
their coordinates, both on the image to be rectified and on a base map, are the most important factors determining the
overall accuracy of the rectification. Operator can manually determine the locations of GCPs on digital image.
Regardless of whether the original or enlarged image, the positional accuracy of the GCP heavily depends on the
experience and performance of the operator. Furthermore, the full automation of rectification is impossible without an
implementation of the automated determination of the GCP(Zhou, 1990). Again, this is extremely difficult since maps
were products of cartographic work. Information of the map has been compressed, generalized, and symbolized, and
thus looks quite different if it is viewed by computer programs
There are two classical methods depending on whether analog or analytic plotter is being used. These two methods are
developing and diversifying under the joint effects of two factors:
e the more and more widespread use of data in digital form;
e the development of information processing systems integrated in new plotters.
Due to the geometric characteristics, it is not possible to use SPOT films directly on classical analog or analytic plotters
designed for the conical geometry of aerial photographs. Recent progress in digital photogrammetry has opened to new
possibilities. Several methods and computer programs for digital triangulation are being developed. Although the block
adjustment phase of aerial triangulation has been automated to a great extent, GCP acquisition phase have practically
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000. 529
