Fig. 2: Spectral band 4 of a Landsat TM mosaic draped over a DEM, showing the area of Jibäl Mugrar. Pixel size as wel 
as DEM cell size is 50 m. Superimposed on the model is the iron anomaly of fig. 4. 
3 IRON ANOMALIES 
Band ratioing is a powerful method for the enhancement 
of mineralogical differences seen in multispectral data 
and their presentation in the form of color ratio images. 
(ROWAN ET AL., 1974). Landsat TM data are even more 
suitable for ratioing due to the increased number of signifi- 
cant spectral bands. In geology as well as in mineral ex- 
ploration, increased iron content of soils or rocks plays an 
important role. Iron is generally enriched in occurrences of 
economic minerals that can be thus emphasized in the 
imagery by the "iron ratio" of TM bands 3/1. Since this 
ratio is, however, highly sensitive to even small amounts 
of Fe, it is by no means sure that an iron anomaly in an 
image is also related to an economically interesting metal 
occurrence. For instance, quartz sand with a thin coating 
of limonitic matter will produce a very strong iron anomaly. 
Two examples of color ratio images are shown in figs. 4 
and 6; Ratios 3/1, 5/7 and 3/5 are coded in R, G and B. A 
20 linear stretch and a 2x enhancement of saturation in 
the IHS color space was applied to the data. Iron enrich- 
ment is indicated by reddish hues in the image: 
Geophysical data like aeromagnetics or gravity, on the 
other hand, provide information on larger bodies with in- 
creased iron content, also if they are not directly exposed 
on the surface. As an example, the residuals of aeromag- 
netic data and gravity residuals of the study area were 
combined in a GIS. High values of both the magnetic and 
the gravity residuals suggest the presence of a geologic 
body with increased iron content. Figs. 5 and 6 show two 
examples of such anomalies. 
428 
In cases where the location of a geophysical anomaly 
corresponds to a high 3/1 ratio, one can safely assume 
the existance of a geologic body with increased iron con- 
tent close to the surface (figs. 2, 3 and 4). On the other 
hand, if a small-amplitude magnetic and gravity anomaly 
shows no relatation to an iron signal on the surface (figs. 
5 and 6), it can be inferred that there is either a deeper. 
seated body or that the anomaly is caused by other ef 
fects like faults or lithologic changes. (GRANT & WEST, 
1965). 
4 GIS CLASSIFICATION 
Digital classification of multispectral data, when applied to 
problems of geology, often yields unsatisfactory results 
due to the low number and wide-band character of thé 
spectral channels. Effects of surficial material further ob- 
scure the original lithologic signal. The major drawback of 
multispectral classification, however, is the fact that pixels 
are classified individually, without taking into account con 
text and existing a priori knowledge. An integrated image 
processing/GIS environment allows the scientist to make 
use of such additional information and to improve the 
accuracy of digital image classification. 
The backbone of a GIS is a relational database in which 
all map information is archived with reference to a com 
mon coordinate system. For the example presented here, 
bands 1, 4 and 7 (plus the ratios 3/1, 5/7 and 3/5) of five 
Landsat TM scenes were geo-referenced to a UTM grid 
and mosaicked, representing an important thematic layer 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996