- 545 - 
± 200 m, and normally only accuracies in the order of ± 300 - 
600 m. 
[ties is 
given 
nited 
il 
, the 
Bologi- 
lic in- 
Lderably 
BO 000 
^graphs 
I be- 
Techno- 
area of 
This accuracy can be met or even exceeded by ERTS-1 
imagery. According to BÂHR & SCHUHR (1974) the mean positional 
error of system corrected ERTS-1 MSS images is ± 171 m, and the 
maximum error amounts to ± 400 m. COLVOCORESSES & McEWEN (1974) 
and KRATKY (1974) report similar values. 
Interpretation of aerial photographs at a scale of 
1 : 50 000 and plotting on a third order stereoplotter results 
in a positional accuracy of ± 25 m, under the condition that 
the control points used are sufficiently accurate. 
Taking into account that the precision of drafting on 
an ERTS-1 enlargement image is in the order of ± 0.5 mm 
(corresponding to ± 100 m in the field at 1 : 200 000), it 
es give 
or field 
follows that in such a case the geological interpretation of 
the satellite image will produce equal or higher positional 
accuracy than field mapping, and that by using aerial photogra 
phic interpretation an even préciser positioning of geological 
of the 
al. 
1 photo- 
STOCK 
K 1972; 
s, a 
ese 
details can be expected. 
For instance, a regionally important lineament in the 
northeastern Tibesti is represented on the existing "Carte géo 
logique provisoire" (WACRENIER 1958) with more than 1 km devi 
ation from its position as shown on the ERTS-1 image. Other 
details are sometimes up to 5 km off their real positions accor 
ding to ERTS-1. 
luated 
Resolution, Detectability, Recognizability 
trans- 
•gements 
The resolution of the ERTS-1 MSS images is in the or 
der of 224 - 316 m, depending on the band used (COLVOCORESSES 
1972). These values refer to photographic resolution and do not 
give the minimum size of objects we can really see on the ima 
ges (ROSENBERG 1971). The detection limits for the smallest ob 
)00. 
)00. 
jects visible on the MSS images of the Tibesti are in the or 
der of 75 m (LIST, HELMCKE & ROLAND 1974) under optimum condi 
tions (contrast, outlines of object, light object on dark 
background). 
;al pro- 
i repre- 
which 
; least 
In theory, when mapping on suitable enlargements from 
satellite images, we come close to the accuracy obtainable by 
geological field work, as far as detection and representation 
of small details under favorable conditions is concerned, since 
areas smaller than 50 m in diameter can also not be shown on a 
map of 1 : 50 000 or smaller. 
But the limit of about 75 m refers only to detectabi 
: avail- 
Bologist 
r than 
lity, and not to recognizability. In this respect the satellite 
image is far inferior to the larger scale aerial photograph. 
The low detection limit of the ERTS-1 image can only be fully 
utilized for geological mapping if we are sure what kind of 
geological feature there is, like sand-filled valley floors or