the temperature and the humidity will cause the aerial photographer 
troubles. 
Liberia has two distinct, climatic seasons of approximately six 
months each: a dry and a rainy season. The dry season, from Novem- 
ber to April inclusive, is ushered in and finished off by violent thunder- 
storms. Marked by a rapid drop of precipitation, this season is charac- 
terized by long periods with a clear sky, most typical during October— 
December. January and February are the driest months, but never 
without some rainfall. At that time, north winds bring high-pressure 
air, coldish, reddish and dusty, from the Sahara desert — called the 
"Dry Harmattan". The rainy season, extending from May to October, 
is characterized by heavy rains, but there is always some sunshine. In 
July and August, the precipitation often drops causing a condition 
known as the "mid-dry" season [2], [4]. 
Although Liberia lies in the equatorial belt, the temperature is not 
extreme. It is more or less uniform throughout the year. The tempera- 
ture ranges from 20 to 35 ?C. The range of humidity is on an average 
65—95 %. I myself have once measured up 100 96! 
2.3 Aerial Photography 1957—58 
As a matter of fact a photogrammetric mapping of tropical areas is 
rarely treated in the technical literature. Therefore, the planning of any 
project might be accompanied with unexpected difficulties. 
Owing to the fact that a special photogrammetric problem is the sub- 
ject of this paper, the planning and the performance of the aerial photo- 
graphy will not be described in detail even though many interesting ex- 
periences have been gained from photographic and cartographic points 
of view. However, it seems to me an unavoidable necessity to make a 
close study of the climate in question because of the different character 
of the tropical conditions. Furthermore, experience shows that a map 
plotting requires a qualified stereoscopic study — if any photographs 
are available — for the planning of the field survey [13]. 
In Liberia, it has been judged that the best period for an aerial photo- 
graphy should be when the rainy season is turning into the dry season 
and, in general, just after the showers followed by clear weather. Later 
on, in January from my own experience, the "harmattan" will set in 
and cause considerable difficulties. Another period is when the dry 
season changes into the rainy season in April—May. However, in re- 
gard to the "harmattan" wind and the fact that the rainy season is 
approaching this period seems to be less favourable. A third period but 
a very short one might be the "mid-dry" season in July—August. 
2.31 Planning of the Photographic Work 
The photogrammetric program of the company at that point 
of time comprised an aerial photography of two alternative lines 
of the railroad, staked out by means of an interpretation of the Ameri- 
6