International Archives of Photogrammetry and Remote Sensing, Vol. 32, Part 7-4-3 W6, Valladolid, Spain, 3-4 June, 1999 
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DEFINITIONS AND TERMS OF REFERENCE IN DATA FUSION 
L. Wald 
Groupe Télédétection & Modélisation, Ecole des Mines de Paris, BP 207, 06904 Sophia Antipolis cedex, France, 
lucien. ■wald @ cenerg .cma.fr 
KEYWORDS: Remote sensing, concept, alignment of information. 
ABSTRACT 
The concept of data fusion is easy to understand. However its exact meaning varies from one scientist to another. A working group, 
set up by the European Association of Remote Sensing Laboratories (EARSeL) and the French Society for Electricity and 
Electronics (SEE, French affiliate of the IEEE), devoted most of its efforts to establish a lexicon or terms of reference, which is 
presented in this communication. A new definition of the data fusion is proposed which better fits the remote sensing domain. Data 
fusion should be seen as a framework, not merely as a collection of tools and means. This definition emphasizes the concepts and the 
fundamentals in remote sensing. The establishment of a lexicon or terms of reference allows the scientific community to express the 
same ideas using the same words, and also to disseminate their knowledge towards the industry and 'customers' communities. 
Moreover it is a sine qua non condition to set up clearly the concept of data fusion and the associated formal framework. Such a 
framework is mandatory for a better understanding of data fusion fundamentals and of its properties. It allows a better description 
and formalization of the potentials of synergy between the remote sensing data, and accordingly, a better exploitation of these data. 
Finally, the introduction of the concept of data fusion into the remote sensing domain should raise the awareness of our colleagues 
on the whole chain ranging from the sensor to the decision, including the management, assessment and control of the quality of the 
information. The problem of alignment of the information to be fused is very difficult to tackle. It is a pre-requisite to any fusion 
process and should be considered with great care. 
1. THE NEED FOR CONCEPT AND TERMS OF 
REFERENCE 
The concept of data fusion is easy to understand. However its 
exact meaning varies from one scientist to another. Several 
words have appeared, such as merging, combination, synergy, 
integration, etc. All of them appeal more or less to the same 
concept but are however felt differently. Several times, the 
word « fusion » is used while « classification » would be more 
appropriate, given the contents of the publication. There is a 
need for terms of reference in the remote sensing community, 
which has been strongly expressed in several meetings. A 
working group, set up by the European Association of Remote 
Sensing Laboratories (EARSeL) and the French Society for 
Electricity and Electronics (SEE, French affiliate of the IEEE), 
devoted most of its efforts to establish a lexicon or terms of 
reference, which is presented in this article. 
This is not the only attempt to set up definitions in data fusion. 
The remote sensing community should not establish terms 
which are also used elsewhere with different meanings. 
Therefore, whenever possible, definitions were adopted which 
are already widely used in the broad scientific community, 
especially that dealing with information. Examples of such 
terms are image, features, symbols, etc. 
Several lexicons have been already set up which have been 
established in the framework of the Defence domain (e.g., US 
Department of Defence, 1991, DSTO, 1994). It was found that 
it is not easy to translate military terms in meaningful words for 
the scientific community dealing with Earth observation: this 
would imply a refinement of the military terms to expand their 
meaning, with a reference to the time-space scales. It was 
concluded that using an existing lexicon is not straightforward, 
and that a new one is required to tackle the specific needs of 
our community. However, we should benefit from this previous 
work as much as possible, and, whenever possible, we should 
use the terms already adopted. 
2. A DEFINITION OF DATA FUSION 
Data fusion means a very wide domain and it is very difficult to 
provide a precise definition. This large domain cannot be 
simply defined by restricting it, for example, to specific 
wavelengths, or specific acquisition means, or specific 
applications. A fusion process may call upon so many different 
mathematical tools that it is also impossible to define fusion by 
these tools. 
Several definitions can be found in the literature: Hall, Llinas 
(1997), Klein (1993), Li et al. (1993), Mangolini (1994), Pohl 
and van Genderen (1998), US Department of Defence (1991). 
They have been discussed by Wald (1998c, 1999). It was felt 
that most of these definitions were focusing too much on 
methods, though paying some attention to quality. As a whole, 
there is no reference to concept in these definitions, while the 
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