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New perspectives to save cultural heritage
Altan, M. Orhan

Leandro Bomaz ( # ), Sergio Dequal ( # )
C) Politecnico di Torino - Dipartimento di Georisorse e Territorio
C.so Duca degli Abruzzi, 24- 10129 Torino
Tel. +39.011.564.7687 / 7601 Fax. +39.011.564.7699
Email: leandro.bornaz@polito.it: sercio.dequal@nolito.it
KEY WORDS: Digital photogrammetry, Analysis, Image, Application, Architectural Heritage, 3D image, laser scanning.
A photo image can be considered a central perspective of the acquired object with good approximation. If the internal and external
orientation of the camera are known, in case of a digital image it is possible to establish the direction in the space of each object point
represented by a pixel in the image. If only one image is available, it is impossible to determine the spatial X,Y,Z position of such
object points, because the simple direction is insufficient: all the points along that direction would give the same image point.
By means of a dense digital elevation model (DDEM) of the acquired object, every pixel (and therefore every direction in the space)
can be associated to the value of distance between the centre of perspective and the object point represented by the pixel itself. In this
way each pixel can be referred to the 3D position of the corresponding object point in an absolute reference system.
The DDEM can be derived from an existing map or by the use of surveying instruments and procedures. Recently, a series of new
instruments, based on the laser technology, have been introduced on the market, giving the possibility to obtain a DDEM in a quick
and cheap way.
The integration of the DDEM geometric data with the image radiometric data suggests a new concept: the “solid image”.
Definition, first results and applications of the solid image was presented in the ISPRS Comm..V, WG V/4 meeting in Ancona, July
2003. In addition, this paper describes the very recent further developments and the future activities on this field.
An image can be considered a central perspective of the
photographed object. If internal and external orientation are
known it is possible, for every pixel of the image, to
determine the direction of its projecting ray in the space. If
only one image is available, that direction is insufficient to
reconstruct the 3D position of the object point (i.e. its X, Y
and Z coordinates). In fact, all the points positioned along
that direction in the space would give the same pixel on the
The recent introduction in the market of laser scanner sensors
in the field of survey instruments allows dense digital
elevation models (DDEM) of the object to be obtained.
The integration of the laser scanner data with the digital
image data can give the possibility of associating a value of
distance to every direction in the space, defined by each
pixel. This distance is calculated as the distance from the
perspective central point (the “taking point”) and the object
point represented by the pixel. If two angles defining a
direction in the space and the above mentioned distance are
known (these 3 figures are called “spherical” or “3D polar”
co-ordinates), it is easy to reconstruct the spatial position (i.e.
the X,Y,Z co-ordinates) of every point represented in the
The traditional RGB radiometric data of the image together
with a 3D model acquired by the laser scanner lead to a new
concept: the “solid image”.
Advantage and potentials of this innovative product are due
to the possibility of:
getting the spatial position of points in the object
reference system;
direct and easy carrying out of correct 3D
measurements on the image;
obtaining a great deal of information in a simple and
rapid way, using the high qiality original images and
any traditional photo viewer software available on the
Based on the solid image, it is possible to foresee several new
applications: a second image of a stereoscopic pair can be
created, a RGB coloured virtual 3D model can be carried out,
2.1 Definition
A “true colour” digital image is made up of 3 matrixes,
named R, G and B, each one containing one of the 3
fundamental colours (Red, Green or Blue).
object space
Figure 1. Relationship between image space and object space
The solid image is defined as a classical digital image that, in
addition, records the distance values of the represented points
from the taking centre. Assuming that the digital image is a
central perspective of the photographed object and the
internal orientation parameters of the image are known, if the