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On the basis of a set of points, the GPT package allows for the 
geometric referencing and transformation of the images 
according to their utilization in the GIS; in particular the 
package was created for the side by side use with Arcview. 
which, as will be described, uses the referencing. 
The control points are measured on the image (indices i. j) and 
supplied to the reference system into which the image must be 
referenced. The numerousness of the points is variable 
according to the geometric transformation which is hoped to be 
adopted and the redundance which is hoped to be obtained in 
the estimation to least squares of the parameters of 
transformation. 
The programs allows for the choice: 
• of the type of transformation 
• of the number of parameters to use (or alternatively, the 
degree of the polynomial); 
• of the dimension of the pixel in the external reference 
system; 
• of the type of algorithm for the interpolation (closest, 
bilinear, bicubic)). 
As a release the program offers: 
• resampled image, with the axes parallel to the reference 
system and with the pixels in the chosen dimension: 
• two files with the parameters of georefercncing. the first in 
proprietary format, the second in ,w from Arcview format. 
The program executes the transformation of the image and 
saves the information on files for its subsequent referencing. In 
this way, the parameters of tranformation arc not saved, but not 
just this: the dimension in X and Y of the pixels and the origin 
of the image in the external system. Such information 
correspond to that which is specified in the "word' files from 
Arcview whose parameters for affine transformations in relation 
to the comers are always zero. 
An interesting characteristic of the program is that of the 
superimposition of the vectors of the residuals on the image 
which allows for the easy individuation of the zones where 
eventual errors might be present. 
fig. 4a 
fig.4b 
figures 4 - Geo Plan Tram program: window fior the choice ofi the 
control points and the superimposition ofi the vectors of the remainder 
onto the image