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pixels), 
Our study is based on the integration of ELECTRE 
(ÉLimination Et Choix Traduisant la RÉalité) methods into a 
raster GIS aiming to resolve spatial decision problems. This 
helps to build a model of the decision maker preferences to 
process it while choosing one of the several alternatives. 
2. METHODOLOGY 
When modelling a real world decision problem using multiple 
criteria decision aid, several problematics (or problem 
formulations) can be considered. [Roy, 1985] distinguishes 
three basic problematics: choice (a), sorting (f) and ranking 
(9. 
Given a set A of alternatives (or actions), the choice (or 
selection) problematic consists in a choice of a subset 4'cA, as 
small as possible, composed of alternatives being judged as the 
most satisfying. 
he sorting problematic consist in formulating the decision 
problem in terms of classification so as to assign each 
alternative a to the appropriate category should rely on the 
intrinsic value of a (and not on the comparison of a to other 
alternatives from A). 
The ranking problematic consists in establishing a preference 
pre-order (either partial or complete) in the set of alternatives 
A. 
The outranking methods deal with only one of the reference 
problematics. Therefore ELECTRE methods are also concemed 
by this rule: 
ELECTRE I: a problematic, 
ELECTRE II 7, 
ELECTRE I: y, 
ELECTRE IV: y, 
ELECTRE TRE A, 
ELECTRE Is: a. 
ELECTRE Tri is a multiple criteria sorting method, i.e., a 
method that assigns alternatives to predefined categories. The 
assignment of an alternative a results from the comparison of a 
with the profiles defining the limits of the categories. This 
method requires the elicitation of parameters ( weights, 
discrimination thresholds, category limits...) in order to 
construct a preference model of the decision maker. 
The main objective of our approach is to reduce the number of 
the alternatives using ELECTRE Tri (sort). In fact, when the 
problematic used is different from sorting (choice or ranking), 
the other ELECTRE should be applied involving the best 
category yielded by ELECTRE Tri (see fig.1). 
The level III of Goraducci [1993] was adopted to build our 
prototype. The user is now provided by a programme and its 
data base which contains both spatial data and multicriteria 
methods base. The multicriteria algorithm is performed either 
in GIS language or in an external routine to be called using the 
same language. This algorithm will be used as a function such 
as the spatial analysis modules. 
Figure 1: ELECTRE approach scheme. 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
| Spatial decision problem | 
v v v 
Set of categories Set of 
and profiles alternatives 
preference Weight- 
indifferenc | | ELECTRE importance 
€ TRI Coefficients 
and veto | ' (AHP 
technique) 
Assignment 
to predefined categories 
. New set of 
B alternatives 
“problematic? > (best category) 
yes 
Yi Yuva 3 
EC Lorre í—— 
: electre :: electre : : electre :: electre :: electre : 
al A Med oS IY iL IR 
  
  
  
  
vv 
| Robustness analysis 9 Recommendation | 
  
3.PRACTICAL PROBLEMS 
The main factors of the agricultural development is irrigation. 
Taking into account of the importance of agriculture and food- 
invoice, scientists constantly look for new soils able to be 
irrigated and then apply a policy of management. This study 
requires a multidisciplinary approach involving topography, 
pedology, geology, geomorphology, agronomy, ecology, 
economy,... 
The study area is situated at 20 km from Maghnia and 150 km 
from Oran. It represents the Northwest extension of the plain 
of Maghnia along the borders between Algeria and Morocco. 
This area is called Zriga; it is 420m higher and is bordered to 
the north by Beni Snassen, to the east by the plain of Maghnia, 
to the south by the plain of Angads (Morocco) and to west by 
the borders between Algeria and Morocco. 
In this study, we have to determine a map 
illustrating three categories: 
e category 1: irrigation suitable land, 
e category 2: fairly suitable land, 
e category 3: not suitable land. 
This is based on the six following criteria: 
e proximity to habitations, 
® to roads, 
e to wells, 
e to hydrographic network, 
e slope, 
e soils types. 
Intemational Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998 87