546 - 
rests of a basaltic lava flow within a sand plain. For the in 
terpretation and recognition of an unknown lithological unit we 
need much, larger areas where grey tone and texture can be de 
termined correctly. 
For instance, the sandstone complex between Bardai 
and Aozou (Northern Tibesti) could be subdivided into four 
different lithological units for the first time exclusively by 
interpretation of aerial photographs (ROLAND 1973), but the 
same differentiation could not be made on the ERTS-1 images. 
The stratigraphic classification, on the other hand, had to re 
ly on fossil discoveries in the field (ROLAND 1971). 
Grey Tone, Multispectral Characteristics, Texture 
Since the satellite image shows very large areas un 
der uniform "real time" conditions in contrast to an aerial 
photographic mosaic of the same region, the observation and 
measurement of the grey tone is much more important than in the 
interpretation of aerial photographs. 
For the interpretation of arid regions, grey tone 
seems to be the most important parameter in the evaluation of 
satellite images. But similar to geological field work, where 
rock color alone is not sufficient for the identification of a 
rock, the grey tone in the satellite image alone does not allow 
us to draw unequivocal conclusions on the kind of rock in ques 
tion. Visual interpretation is also rendered more difficult by 
the fact that areas of equal grey tone (corresponding to a cer 
tain rock unit) are often small, and differences in grey tone 
minute. 
Here differences in the spectral remissivity of rock 
surfaces are of help, especially where soft rocks are concerned. 
Fig. 1 shows a sand plain in the eastern Tibesti, where some 
sands show up much darker on band 7 (0.8-1.1 pm) than for in 
stance on band 5 (0.7-0.8 pm). In other cases, the opposite 
effect can be observed. What reasons beside different humidity 
contents are responsible for this effect will have to be 
cleared up by laboratory investigations. 
Since we cannot get a stereoscopic view of most satel 
lite images as yet, texture becomes a very important factor in 
the interpretation of orbital images. Texture depends on scale 
(CARTER & STONE 1974); many details that can be observed as 
separate "signals" in an aerial photograph, are squeezed toge 
ther into an often characteristic texture in the small scale 
satellite image. Quantitative analysis of texture patterns will 
probably be of tremendous help in the identification of rocks 
from satellite images. Somewhat simplified it could be said 
that detectability depends mostly on differences in grey tone 
(contrast !), and recognizability of differences in texture. 
Fig. 2 shows some typical examples of textures of 
different rock units in the Tibesti.