It may be added that the exposure of the pancro-film was mostly 
1/200 sec. at the aperture of 1:5,6. By using the infra-film, the quite 
unexpected but important observations were made that this film was 
more sensitive. Thus, an exposure of 1/300 sec. was generally used at 
an aperture of 1:8. 
Thanks to a well-trained crew and a photogrammetric staff who also 
served the laboratory, the photographic result of the pictures has been 
first-rate quality altogether. In all, about 1 300 photographs were taken 
of which the infra pictures amounted to 47 %. 
The group could finish the commission in the middle of February 
1958. 
2.33 Field Survey 
Due to the lack of a geodetic net as mentioned, the only basis for a 
map plotting at that time was formed by the American maps, i. e. the 
mosaics at the scale of 1:20 000. Thanks to these maps, LAMCO had 
the means of making a more detailed mapping that in all probability 
should be asked for in a short time by the engineers. On discussing 
the accuracy needed, however, it must be kept in mind that the mosaics 
have been carried out at the scale of 1:40 000 before the reproduction in 
1:20 000. Besides, as far as it concerns the mountainous areas, consi- 
derable discrepancies are to be expected. 
Anyhow, this map formed the only way to get an average scale of 
any photogrammetric map at that time. In a later phase of the develop- 
ment of the project a geodetic net had to be a necessity in accordance 
to the increased requirements by the engineering people. 
Consequently, the only course to get a height measurement involved 
a levelling in a barometric way, carried out by LAMCO in 1956—58. 
As far as the vegetation rendered it possible, the control points have 
been selected in regard to the requirements of the photogrammetric 
treatment. 
2.34 Photogrammetric Interpretation 
By means of the aerial photographs taken in 1957—58 a close inter- 
pretation study of the railroad line, preliminary staked out as mentioned 
above, had to be prepared. Using mirror stereoscopes the aim has been 
to improve the first staking by means of the up-to-date pictures at the 
two scales of 1:25 000 and 1:8 000. So, a profile of the line was carried 
out in the Wild A 6 and A 8 instruments. 
2.35 Photogrammetric maps 
The plan of the photogrammetric map plotting had been approved by 
LAMCO as follows. At the planning stage, the American mosaics com- 
bined with a barometric levelling had to be considered as satisfactory. 
Thus, a general map of the Nimba Area at the scale of 1:20 000 had