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the highland zone, 400—600 m above sea-level, follows from the hinter- 
land to the boundary of Guinea, 
Starting from the port at Lower Buchanan and having passed the 
coast zone, partly swampy and interspersed by smaller streams, the 
railroad line has to cross St. John River twice within the hinterland 
zone. Here the vegetation is classified as evergreen tropical rain forest. 
The transition to the highland zone is very hilly and untraversable. 
From vegetational points of view this zone is classified as the high 
forest areas — with trees up to a height of 80 m — including the Nimba 
range area. 
2. Photogrammetrical planning 
When this project had to be planned in 1956 it was decided that 
photogrammetric methods should be used for laying out the rail- 
road line, as well as for the planning of the mining and the harbour 
areas. Besides, maps were also needed for other construction purposes. 
Thus, the detailed planning of the railroad — undertaken by 
SENTAB — commenced in 1957 by means of an interpretation 
followed by a stereo-measurement, using the existing photogrammetric 
material of Liberia taken by the Americans in 1952—53. 
21 Aerial Photography 1952—53 
According to an agreement between the governments of Liberia and 
the United States in 1950, an aerial photographic and mapping project 
had to be carried out. Thus the U. S. Coast and Geodetic Survey was 
authorized to conduct the project which was carried out during the pe- 
riod 1951—56 in close co-operation with the Liberian Cartographic 
Service that was organized these years. 
The project begun with a series of 20 second-order astronomic posi- 
tions, established throughout Liberia to furnish the basic control of a 
Shoran-controlled photography and to serve as reference stations for 
future land surveys. At four of this sites the Aero Service Corporation, 
Philadelphia, set up shoran stations for control of the aerial photo- 
graphs taken during the years 1952—53. The cameras then used were 
Metrogon, c:6,3/6", and the flying height was 6 000 m above ground, 
resulting in a scale of photographs of 1:40 000. To supplement the pho- 
tographs, the Liberian Cartographic Service had taken some additional 
aerial photography [5], [15]. 
The mapping program comprised controlled mosaics on a scale of 
1:20 000 and planimetric maps on the scales of 1:125 000, 1:500 000 
and 1:1 000 000. 
2.2 Meteorological Conditions 
In order to facilitate the understanding of the problems that will face 
people who have to carry out an aerial photography in the Tropics, I 
should like shortly to give an idea as to the climate in general. Especially 
5