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GEOPLANTRANSF: A SOFTWARE FOR THE GEOREFERENCING OF DIGITAL IMAGES
BY USING PLANE TRASFORMATIONS
Francesco Guerra*, Caterina Balletti**, Davide Miniutti***
*3 rd year Course of PHD
**2 rd year Course of PHD
***1”* year Course of PHD
DIIAR Dipartimento di Ingegneria Idraulica, Ambientale e del Rilevamento
Sezione Rilevamento - Politecnico di Milano
P.zza Leonardo da Vinci 32 - 20133 Milano
e-mail: guerra2@brezza.iuav.unive.it - balletti@brezza.iuav.unive.it - miniutti@cidoc.iuav.unive.it
KEY WORD: GIS, georeferencing, digital images, plane transformation, free software
Abstract
In this paper the working principles and some applicative examples of the software realized by the authors for the georeferencing of
digital images are presented.
The program allows for the calculation of the parameters for transformations between two digital images or between an image and a
cartographic system. The general form is used for the polynomial plane transformation for the crossing over from one system (o; x,
y) to another system (0; X, Y) with the possibility, on the part of the user, to utilize a smaller number of parameters and realize a
projective, affine, or conform transformation, among others. The estimation of the parameters is to the least squares and the
program realizes the superimposition of the vectors of the residuals onto the image, for the immediate evaluation of the results of the
transformation. On the basis of the estimated parameters (which are m any case saved in the file) it is possible to resample the image
in the destination reference system according to three methods: nearest neighbor assignment, bilinear interpolation or bicubic splines.
Some of the applications of this software package will be illustrated. The software is written in Borland Delphi
language, and distributed in freeware way.
Digital images and GIS
The digital images are widely used within the GIS according to
two different aspects: first, as cartography as a basis and
second, as an organization for thematic data.
In the first case it can deal with either scannings of existing
cartography for which a vectorial form is not available or
photographic quality' images which when correctly treated, can
substitute for the base map. This is the case, in architecture, for
example, with rectifications of the façades of buildings for
which a vectorial restitution is not to be done.
The second case is one in which the images are utilized to offer
a description of phenomenona through which quantity or
punctual or associated to unit areas, that is quantities organized
in matrix form. The typology of these cases is extremely vast.
It is likewise true that most of the time the two cases described
coincide when, from these photographic maps, it is possible to
obtain qualitative information.
The use of the digital images is fundamental insofar as it allows
an easy use of the data typical in spatial analysis. Premised in
such operations is that the digital images are brought back to a
known geometry, utilizing specific procedures of referencing
and calibration.
The software presented here resolves a small part of these
problems, using a polynomial plane transformation whose
number of parameters is chosen by the user.
Introduction to the software
The GeoPlanTransf program is the development of some
software produced by the authors in the field of geometric data
processing of digital images. In the course of the recent years
infact software has been developed, the first for affine
transformations of digital images, the second for the
rectification in the close range photogrammetry field.
The program for affine transformation was realized for the
internal orientation of digital images as a module of the package
for digital orthophotoprojection (Orthodigit). The algorithms
utilized in this software implement the traditional equations:
X = a ] + b l x + c l y
Y = a 2 + b 2 x + c 2 y
The RDF program on the other hand was bom to meet didactic
needs, provide for the large request for photogrammetric
surveys for rectification on the part of professors and students
involved in restoration courses. The software, distributed in free
form, has diminished the pressure placed on the research centers
of the university while at the same time allowing the students an
approach to the survey not only as users but also as maker. The
equations used are those of the projective transformation in the
form commonly used in photogrammetry and which derives
from the semplification of the collinear equations in the case of
the Z constant.
Y _ <3 i x + a 2 y+a 3
c t x+c 2 y+1 2
Y _ b x x + b 2 y + b-
c^x + c 2 y+1
The software presented here was bom from the experience of
the two aforementioned software and it unites them and
strenghthens their characteristics according to the referencing
and the calibration of the digital images within the GIS. This is
bom from within the research work done by the authors for their
PHD course work.
The destination of the GPT software is logically teaching and
research given on the one hand the environment in which it was
bom and on the other its form of circulation. Beyond the fact
itself that, as in its predecessors, it allows for the rectification of
images, and renders this rectification immediately utilizable for
the teaching of the survey
