TRUE ORTHOPHOTO OF THE WHOLE TOWN OF TURIN 
Sergio DEQUAL, Andrea LINGUA 
Dipartimento di Georisorse e Territorio, Politecnico 
Corso Duca degli Abruzzi n. 24 1-10129, Torino (TO), ITALY 
email: sergio.dequal@polito.it 
andrea.lingua@Dolito.it 
Commission VI, WG VI/4 
KEY WORDS: Digital Orthorectification, DEM/DTM, Aerial Photogrammetry for Cultural and Natural Landscapes, True 
orthophoto, Interpolation 
ABSTRACT: 
The wide diffusion of modem GIS/LIS instruments requires new geomatical techniques that allow also to unskilled operators the 
use of such powerful tools. Digital orthophoto is a cheap and efficient product able to integrate GIS/LIS applications with a 
rigorous photographic representation of urban areas. 
In large-scale mapping applications, especially where complex architectural objects have to be represented, the problem becomes 
complicated because there are many discontinuity lines (break-lines) and hidden areas. This requires a more sophisticated 
orthoprojection procedure, called “true orthophoto”. 
In recent years, the authors have developed and presented (CIPA, 2001) an original procedure to produce true orthophotos using a 
“dense DEM” (DDEM) and all the images available in the photogrammetric block (“multi-image” procedure). A software package 
able to resolve this purpose has been implemented in Visual Fortran and optimised for practical mapping application. It consists in 
two modules: 
ACCORTHO (=ACCurate ORTHOphoto), to produce rigorous digital orthophotos starting from multiple images and a 
DDEM; 
GENEDDEM (GENEration of a DDEM), able to build a dense DEM (suitable for ACCORTHO) from a 3D digital map, 
using sophisticated interpolation techniques. 
This paper describes in detail the structure of the GENEDDEM and a practical application of both the programmes for a relevant 
mapping project, that is still in progress: a true orthophoto of the whole town of Torino (about 5000 ha), in scale 1:2.000, 
containing many complex architectural objects (Mole Antonelliana, the Gran Madre Church, the Holy Shroud Cathedral). 
1. INTRODUCTION 
The diffusion of technologies inherent to Geographic 
Information Systems (GIS), especially in urban areas, has led 
to new instruments for the solution of a series of problems 
connected to new duties that municipality administrations have 
to perform: the necessity, established by law, of rationalising 
the competences of the various territorial organisations in a 
cartographic ambit and the production of suitable instruments 
to interchange geographic information. 
GIS at a local scale require representations of the municipality 
territory in a large-scale digital map, often in three- 
dimensional form (3D). 
The digital orthophoto constitutes an efficient and low-cost 
product that can be used to complete the contents of a GIS with 
a both metric and photographic representation of the territory, 
that helps inexperienced users in interpreting the territorial 
objects. The geometry of this particular photographic map is 
obtained through the orthogonal projection of each pixel of the 
image, representing whether natural or man-made objects, onto 
the cartographic plane in such a way that the original 
prospective representation, such as the aerial photograph, is 
transformed into an equivalent image, metrically correct: 
measuring co-ordinates, angles, distances, areas on the 
orthophoto gives correct results, exactly as on a map. 
If the surface of the territory is continuous (smooth) it can be 
efficiently described using a regular grid (DEM) of the 
assigned points in which the sides of the mesh have 
dimensions that vary from 5 to 50 m according to the 
representation scale (usually 1/200 of the scale denominator). 
In this case it is very easy to make the orthophoto: numerous 
adequate commercial software programs are available that can 
guarantee acceptable precision in line with the relative 
cartographic tolerances. Unfortunately, continuity does not 
exist in urban centres: a surface that is covered by artificial 
objects (buildings, infrastructures, bridges, etc.) is surely not 
smooth and cannot therefore be modelled with a classical 
DEM. In these cases, it is necessary to use more refined 
solutions. The authors [Dequal, Lingua, 2001] have recently 
proposed and practically developed a procedure for accurate 
orthoprojection (ACCORTHO) that is based on a “dense 
DEM” (called DDEM = Dense Digital Elevation Model) and 
which is developed in Visual FORTRAN language. 
The Municipality of Turin, in collaboration with CSI-Piemonte, 
falls into this context in the ambit of development programmes 
of its GIS (which is made up on a digital cartographic base in a 
1:1000 scale). The Municipality saw the opportunity of 
integrating this instrument with digital colour orthophotos of 
the territory and drew up a research contract with the Geo 
resources and Land Department of the Politecnico di Torino. 
This project, of remarkable dimensions (the territory belonging 
to the municipality covers about 12.000 hectares) has led to a 
further development of the ACCORTHO software through a 
series of interventions that were made to improve the