278 
Another interesting image is produced by the ratio- 
ing bands 3 and 4. Band 3 coincides with a chloro 
phyll absorption band while the response of band 4 
is controlled by the physical structure of the meso- 
phyll layer of leaves (Townshend 1984). The result 
ing image displays vegetation as dark tones (Fig. 7b) 
Three phenomena stand out in this image: the tempora 
ry vegetation along the wet ephemeral channels, agri 
culture along old channels in the southern fans and 
the oasis of El Guettar, a spring line along the dis 
tal end of the fans. 
Multivariate enhancement techniques reduce the var- 
Table 4. Eigenvectors and eigen-values derived from PCA of 6 reflective TM bands for test site. 
Figure 7. El Guettar: a)5/7 ratio b)3/4 ratio, 
Figure 8. El Guettar: a) PC3 b) PC4. 
SFECTRAL BANDS 
PC 
1 
2 
3 
4 
5 
6 
eigenvalue: 
1 
-.252 
-.210 
-.352 
-.353 
-.675 
-.430 
93.8 
2 
-.569 
-.315 
-.411 
-.283 
.502 
.270 
4.3 
3 
.345 
.148 
.120 
-.795 
-.106 
.447 
0.9 
4 
-.264 
-.017 
-.033 
.355 
-.526 
.724 
0.6 
5 
.619 
-.181 
-.729 
.187 
.032 
.119 
C. 2 
6 
-.196 
.895 
-.396 
- .006 
.031 
-.041 
0.0 
* Eigenvalues (x 100 per cent) indicating the proportion of total statistical variation contributed by each 
principal component.