Page 198 - Geochemical Anomaly and Mineral Prospectivity Mapping in GIS

200 Chapter 7

Fig. 7-6. Continuous rating scale for pairwise comparison of relative importance of one criterion
versus another criterion with respect to a proposition (adapted from Saaty, 1977).

Eastman et al., 1995). De Araújo and Macedo (2002), Moreira et al. (2003) and
Hosseinali and Alesheikh (2008) provide case applications of the AHP to derive criteria
or evidential map weights for mineral prospectivity mapping. The application of the
AHP is useful not only for binary index overlay modeling but also for multi-class index
overlay modeling (see further below) and fuzzy logic modeling.
The method of deriving criteria weights via the AHP involves pairwise comparisons
of criteria according to their relative importance with respect to a proposition. The
method adopts a 9-point continuous pairwise rating scale for judging whether Criterion X
is less important or more important than Criterion Y (Fig. 7-6). The relative importance
rating is read from the either the left or right extremity of the scale, depending (a) on
which extremity each of two criteria is positioned and (b) on which criterion is compared
to the other. In addition, the importance rating of one criterion is always the inverse of
the importance rating of the other criterion. For example, if Criterion 1 and Criterion 2
are positioned at the left extremity and right extremity, respectively, and if Criterion 1 is
‘weakly’ less important compared to Criterion 2, then the relative importance rating of
Criterion 1 compared to Criterion 2 is 1/3. It follows that the relative importance rating
of Criterion 2 compared to Criterion 1 is 3. Reversing the positions of Criterion 1 and
Criterion 2 does not change their relative importance ratings. Thus, if Criterion 1 and
Criterion 2 are now positioned at the right extremity and left extremity, respectively, and
if Criterion 2 is ‘weakly’ more important compared Criterion 1, then the relative
importance rating of Criterion 2 compared to Criterion 1 is 3. It follows that the relative
importance rating of Criterion 1 compared to Criterion 2 is 1/3. The pairwise relative
importance ratings of all possible pairs of criteria are then entered into a pairwise
comparison matrix.
For the epithermal Au prospectivity recognition criteria in our case study area, the
pairwise comparisons of relative importance of each criterion (Table 7-I) are based on
the results of the spatial analyses in Chapter 6. Proximity to NNW-trending
faults/fractures is considered to be between moderately and strongly more important than
proximity to intersections of NNW- and NW-trending faults/fractures; thus, a rating of 6
is given to the former. This is because, in the case study area, the NNW-trending
faults/fractures seem to have more influence in the formation of dilational jogs (Fig. 6-
16), which are known to be favourable sites for mineralisation. Proximity to NNW-
trending faults/fractures is considered moderately more important than proximity to NW-
trending faults/fractures; thus a rating of 5 is given to the former. This is because, in the