base. The Arpenteur 
s an explicit input to 
hange data between 
he same software and 
n the same team for 
ense availability, the 
simply because of the 
hange or share data is 
le and is a concern of 
based tool [Drap, 
t teams located in 
to cooperate to some 
already begun to build 
h the interoperability 
ave already begun in 
rammetric field. For 
System (GIS), the 
pecifications to define 
or more sophisticated 
on of coordinates 
st is the one that deals 
odelling: the VRML 
'ards standardization : 
is also attempting to 
ML Schema (see their 
idex.html) 
ge dedicated to 
ie. W3C consortium 
ed many technical 
ucture as a W3C 
bility. This means 
he Web accessible to 
are environment that 
use of the resources 
.0 Recommendation 
first step towards the 
community to design 
ls and integrate them 
ture based on XML" 
e XML specification 
describes "a syntax 
ly used international 
Generalized Markup 
ed and corrected) for 
"XML documents are 
, which contain either 
ita is made up of 
ter data, and some of 
a description of the 
ucture. XML provides 
he storage layout and 
lefines the concept of 
med document simply 
ements whereas valid 
documents respects some constraints on document 
structure. (Extensible Markup Language (XML) , 2000) 
The XML Schema specification (XML Schema Part 0, 
Partl, Part 2, 2001) is built atop the solid foundation 
provided by XML and provides a way to define in a 
separated document (written itself in XML) the structure 
of entities, their datatypes, the relationships between 
these entities and constraints as well. The so-called 
schema gives thus the conditions for a given document to 
be declared valid with respect to this schema. 
4. DESCRIPTION OF PHOTOGRAMMETRIC 
MODELS 
Since the beginning of the year 2002, the Arpenteur team 
has begun to generalize the use of XML as the core tool 
to express photogrammetric projects and results. This 
means that all projects and results can be expressed with 
XML as a basic storage, although other solutions as SQL 
data bases can also be used for the data storage. 
A first version (validity check) has been developed and is 
currently used in version 3.0. This version makes use of a 
DTD (Data Type Definition file) that defines the validity 
of Arpenteur photogrammetric projects written in XML. 
A DTD however does neither permit the definition of 
complex types nor provide the ability of performing 
uniqueness checking or referential integrity validation. 
But XML Schema does. The definition proposed here is 
thus presented through XML Schema for public review 
and should be understood as a design step in order to 
provide our community a common way of exchanging 
data and projects. This will be followed after eventual 
corrections by a first implementation. We will try to 
respect a process of open specification-development 
discussions. Usefull related links and forum addresses 
can be found at the end of this paper and free tools for 
reading or validating data documents and projects will be 
made available as well. 
As a short paper like this one is not the right place to 
show a complete specification, (available on our web 
site), we will however give the understanding of how is 
structured a Photogrammetric Model in our schema. 
4.1 XML Schema for photogrammetric models 
The purpose of a this schema is to define a specialized 
class of XML documents that deals with 
photogrammetric data structures. To do that we propose a 
particular schema for documents referring to 
photogrammetric models, easy to by different 
applications or working teams. 
As the reader will convince himself in a short delay, the 
schema and the resulting documents will be human- 
legible and reasonably clear (this is one of the goals of 
the XML specification). As said, the complete schema 
will not be given here. In place we will present its main 
elements together with an example that illustrates how 
projects conforming to this schema will look like. Let us 
start by considering how the main Photogrammetric 
Model (Model for short) element is written. 
  
Table 2 — Excerpt of model definition 
4.2 The "model" element 
The 'model' element consists of seven subelements, model- 
description, orientation-list, photography-list, point2D- 
lists, point3D-lists and controlPoint-list and three attributes, 
version, author and geoSystem. Each subelements might 
contain other subelements, and so on until a subelement 
contains a data rather than any subelement. Elements that 
contain subelements or carry attributes are known as complex 
types, whereas elements that contain data are known as simple 
types. 
The types' definition is an XML Schema that enables type 
checking during document parsing. It allows also the use of a 
query language (Xpath) to assert additional constraints, like 
uniqueness or referential integrity. For instance, the model 
element defines an Identity constraint on photographs : each 
photography referenced in a point2D-list must exists and be 
already declared in the list of photographs : photography-list, 
in which it must be unique. Other similar constraints 
(constraints are not shown in the excerpt of the document) 
apply to referential and their use in point3D-lists. 
Attributes of the model element define the version of the 
model which is a simple type, roughly a string, that permits 
recording changes in the model. It is mainly a facility offered 
to users to be able to agree on the right version of the 
document to be exchanged. The author attribute (optional) 
gives an indication on the source of the document and the last 
attribute, geoSystem indicates the type of coordinate system 
that will be used in the model. GeoSystem is a simple type 
based (restriction of) on the geoSystem type defined in the 
X3D draft specification 
(http://www.web3D.org/TaskGroups/x3d/) It has a default 
value of "LSR" meaning Local Space Rectangular. The only 
other coordinate system currently defined is "GCS" (Global 
Coordinate System), but other types may be added in the 
future as well as types derived from the OpenGIS 
specifications. 
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