INTRODUCTION 
The objective of all geological mapping activities is 
to enlarge the knowledge of the geological setting in a given 
area. This objective must be met, normally, within a limited 
time and a limited budget. While the classical geological 
approach had to rely on direct observation in the field, the 
use of aerial photography made it possible to extract geologi 
cal information in an indirect way. By aerial photographic in 
terpretation much larger areas could be studied in considerably 
shorter time, but in areas like the Tibesti Mountains (80 000 
sq.km) that are covered by as much as 3 700 aerial photographs 
at a scale of 1 : 50 000, the limitations of this method be 
come obvious, too. In contrast, NASA's Earth Resources Techno 
logy Satellite 1 (ERTS-1) is covering nearly the total area of 
the Tibesti Mountains with only 8 frames. 
But does the interpretation of satellite images give 
the same results as aerial photographic interpretation or field 
work? 
After completing a geological interpretation of the 
Tibesti Mountains, based on the ERTS-1 imagery (LIST et al. 
1974), and with about 4 300 sq.km mapped by conventional photo 
graphic interpretation at a scale of 1 : 50 000 (LIST & STOCK 
1969 ; ROLAND 1 973, 1974 , 1975; SALAHCHOURIAN 1975; STOCK 1972 ; 
TEHRANI 1975) in addition to ground truth investigations, a 
first account of the advantages and disadvantages of these 
different methods can be given. 
The following photographs and images were evaluated 
for this investigation; 
1. ERTS-1 imagery (MSS, bands 4-7); positive trans 
parencies (1 ; 1 000 000), and paper enlargements 
(1 ; 200 000 ). 
2. GEMINI and APOLLO color photographs. 
3. Aerial photographs, panchromatic, 1 ; 50 000. 
4. ’Aerial photographs, panchromatic, 1 ; 20 000. 
GEOLOGICAL INFORMATION 
Positional Accuracy 
Positional Accury is basically not a geological pro 
blem. But geological data are valuable only if they are repre 
sented in a geological map with the same accuracy with which 
they are observed in the field, or which corresponds at least 
to the accuracy of the topographic base used. 
Since topographic maps of the Tibesti are not avail 
able at a scale larger than 1 : 200 000, the mapping geologist 
in the field cannot expect a positional accuracy better than