redundant efforts. As the use of XML formalism has
reached a certain level, it appears now as a rational
choice
Example: exporting archaeological surveys in XML
Once the photogrammetric data are formalised with
XML, we start to store the plotting result data in the same
way. ARPENTEUR has a particular way to manage
resulting data, not only as geometrical data but also in a
semantic and topologic way.
This approach was implemented in an underwater
photogrammetric survey for an Etruscan deep wreck
discovered near Marseille two years ago. In this context
we develop a special tool for surveying amphora, using a
theoretical and geometrical model in order to complete
the amphora lacked geometry (due to partial vision or
partial destruction).
An XML file is generated in order to manage all
measured data concerning the survey.
For more information about this work you can refer
[Drap, Long, Durand, Grussenmeyer, 2001-A], and also
[Drap, Bruno, Long, Durand, Grussenmeyer, 2002]
3. A MODEL FOR PHOTOGRAMMETRIC
PROJECTS
Since the beginning of the ARPENTEUR project, the
need to work in a team and with other teams together
with an object oriented programming language has
obliged us to design our internal tools in a modular
fashion, with embedded concepts closely related to
general uses in the photogrammetric field, e. g the
definition of a so-called Model for photogrammetric
projects. To describe this model and reach the goal of
interoperability, we use XML and XML Schema
languages. The goal of this section is then to show how a
photogrammetric model would be formalized using these
languages and to propose it as a first step toward a
standardized way to describe photogrammetric data.
3.1 The need for a model — Why a Model ?
We all know that when realizing some photogrammetric
work, we are dealing with large amount of data,
thousands of measures, hundreds of points, etc. All these
geometric entities need to be clearly assigned to some
usage. A structured representation of data is then a basic
requirement to build a photogrammetric software
application.
A model is a structured feature that is built to drive a
system towards some possible solution of an issue. The
concept of Model defines the ‘world’ of a problem. It
clearly establishes what is inside and belongs to the
universe of the problem and what should be left outside
as non meaningful for it [Lemoigne, 1990]. Such issues
in photogrammetry will be surveys, 3D representation or
construction of an Information System. For instance, to
build an "Information system on heritage conservation"
[Camara, Latorre, 1997], structuring of data has been
made through a relational data base. That means the
existence of a data base schema (a kind of model). In this
case, the model remains implicit because it is wrapped in
the underlying structure of the data base. The Arpenteur
project is using the concept of model as an explicit input to
work with data.
3.2 Interoperability
Another point was the ability to exchange data between
different teams. Not all the teams use the same software and
more than one software my be used in the same team for
different reasons like : the software license availability, the
preference for a given functionality or simply because of the
knowledge of the user. The ability to exchange or share data is
also known as the "interoperability" issue and is a concern of
the Arpenteur project as a web based tool [Drap,
Grussenmeyer, 2000].
Interoperability would allow different teams located in
different places to exchange data and to cooperate to some
common goal. Other organisations have already begun to build
standards or recommendations to reach the interoperability
through different systems. Such efforts have already begun in
fields that are closed to the photogrammetric field. For
instance, in Geographic Information System (GIS), the
OpenGIS Consortium has built several specifications to define
basic data structure [OpenGIS, 1999] or more sophisticated
services, for instance: transformation of coordinates
[OpenGIS, 2001]. Another field of interest is the one that deals
with 3D representation and reality modelling: the VRML
consortium has lead its specification towards standardization :
see [VRML, 1997]. This organization is also attempting to
develop its new standard by the use of XML Schema (see their
draft version at
www.web3D.org/TaskGroups/x3d/X3dIndex.html)
3.3 XML and XML Schema : a language dedicated to
structured data
Since early stages of the Web, the W3C consortium
(http:// WWW.w3c.org/) has developed many technical
specifications for the Web infrastructure as a W3C
commitment to promote interoperability. This means
encouraging universal access to make the Web accessible to
all, semantic Web to develop a software environment that
permits each user to make the best use of the resources
available on the Web and a Web of Trust.
Among these efforts, "the XML 1.0 Recommendation
(published in February 1998) was the first step towards the
next generation Web, allowing each community to design
languages that suit their particular needs and integrate them
harmoniously into a general infrastructure based on XML"
(http:// WWW .w3c.org/Consortium/) The XML specification
(Extensible Markup Language, 2000 ) describes "a syntax
created by subsetting an existing, widely used international
text processing standard (Standard Generalized Markup
Language, ISO 8879:1986(E) as amended and corrected) for
use on the World Wide Web". For that, "XML documents are
made up of storage units called entities, which contain either
parsed or unparsed data. Parsed data is made up of
characters, some of which form character data, and some of
which form markup. Markup encodes a description of the
document's storage layout and logical structure. XML provides
a mechanism to impose constraints on the storage layout and
logical structure". The XML language defines the concept of
well-formedness and validity : a well-formed document simply
respects the XML way of structuring elements whereas valid
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