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GEOLOGICAL MAPPING IN ARID REGIONS OF AFRICA USING SATELLITE DATA -
INTEGRATION OF VISUAL AND DIGITAL TECHNIQUES
Franz K. List, Wolfgang Koch and Mohammad H. Salahchourian
Freie Universitát Berlin, Institute of Geology, Geophysics and Geoinformatics
D-1000 Berlin 46, Malteserstr. 74-100, Germany
ABSTRACT
Landsat-MSS data were used for preparing a geological map of entire Egypt at a scale of 1 : 500 000 in 20 sheets. This
endeavor comprised production of optimized imagery for visual interpretation and field checks, as well as creation of digital
mosaics of the whole country for base maps. Geometric control points were obtained during geologic field work from Transit
satellites. Landsat-TM data, utilized in a present project in northeastern Sudan, provide improved spectral discrimination and
higher spatial resolution. Color-ratio imagery is used for detection of gossans and gold-bearing quartz veins. Satellite, field, and
elevation data are input into a GIS together with geophysical and petrologic data. Classification of lithology and subsequent map
production is significantly improved by the integration of these information levels.
1. INTRODUCTION
Since the launch of ERTS-1 opened "a new window on our
planet" (Williams & Carter, 1976) in July, 1972, geologists
have made extensive use of the geometric and thematic
information provided by the digital image data of Landsat
MSS and TM. Landsat MSS data with their coarser resolution
are a good choice for regional geologic analysis, both for
structural as well as for lithologic investigations. TM data, on
the other hand, provide improved information on spectral
anomalies and smaller terrain features. From the beginnings
of remote sensing, visual interpretation techniques familiar
from "photogeology" were used for deriving information from
the image data. With the growth in computer power and
availability during the last decade, digital data processing sig-
nificantly increased the quality of image representation, clas-
sification and geometric accuracy. In this way, remotely
sensed data can be operationally employed for regional geo-
logic mapping and exploration, especially in arid and little ex-
plored areas.
2. MAPPING IN EGYPT
After preliminary studies on applications of early Landsat-1
imagery to small-scale geologic interpretation in the Tibesti
Mountains, Chad, resulting in a satellite interpretation map
(List, Roland & Helmcke, 1974; List, Helmcke, Meifàner et
al., 1978), a pilot project was started in Egypt using stan-
dard image interpretation methods for preparing a first geo-
logic map of southwestern Egypt at 1 : 500 000 scale (List
et al., 1982, 1984). In this project, the image data used for
interpretation were MSS standard products in the form of
photographic data.
Any interpreter who has worked with these early standard
products has made the experience that the image quality was
not really good enough for proper identification of geologic
details or minor spectral differences in lithology. Also the
image geometry was not up to map standards even at small
scales.
Thus in 1981 a mapping project was started in cooperation
with CONOCO with the intention to produce a geologic map
of entire Egypt at 1 : 500 000 that would conform to inter-
national mapping standards. The map was to consist of 20
sheets covering an area of 2° by 3° each. The following ob-
jectives were set for the final map:
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® To produce geometrically corrected digital mosaics as a
map base guaranteeing topographic accuracy;
® To adopt a format of the map sheets conforming to the in-
ternational World Map grid;
® To carry out adequate field work to be able to properly
classify and correlate the sedimentary and basement
rocks;
® To perform the geologic interpretation and compilation on
high-quality color composite imagery for best results; and
€ To complete the entire project possibly within 6 years.
2.1 Geologic setting and map philosophy
Egypt is an arid country with an area of more than 1 million
km? (see fig. 1). Only 396 of the land surface, namely the
Nile valley and the Delta, are covered by vegetation.
SES Mediterranean :*.
Fig. 1: Sketch map showing Egypt and the actual
study area in Sudan.
