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investigations also serve to collect data, which can be used for determining 
priorities of development. A general soil survey, indicating the most important 
and most extensive soil associations, with a resultant soil map on a scale of 
1 : 100,000 or 1 : 200,000, will be sufficient for this phase. 
Such a soil survey can easily be made by means of aerial photo interpretation 
in combination with terrain investigations. Procedures for this type of work 
have been described in detail, in chapter 11, appendix A of the Manual of 
Photographic Interpretation published by the American Society of Photo- 
grammetry in 1960. 
Soil mapping is generally done by aerial photo interpretation techniques. 
Soils are studied in the terrain in a number of sample areas, covering the main 
soil associations as shown on the soil map. The soil studies in the terrain and 
the soil analyses to be made in the laboratories are made for information 
regarding: 
a. the soils occurring in the project area, 
b. the evaluation of soil conditions, 
c. the estimation of real potentialities, 
d. the predictions for future agriculture, crops, yield, etc. 
The main purpose of the soil map in this phase is to show what soil associa 
tions occur, where they are located and how they are associated. Although 
the accuracy of the soil boundaries should be higher than the exploratory soil 
map of phase 1, there is, even in phase 2, no need for very accurate and de 
tailed soil boundaries. For the same reason relatively small areas (100 ha and 
smaller) can be omitted, and consequently a general soil map 1 : 100,000 or 
1 : 200,000) mainly based on aerial photo interpretation as far as plotting of 
soil boundaries is concerned, will be sufficient. 
If a few potential areas have to be investigated in order to select the most 
suitable one for further development, it is often worthwhile stopping the field 
investigations as soon as it becomes evident that one region is much better 
than the others. A short interim report to the responsible authorities will 
provide the possibility of concentrating further investigations in the best region. 
There is no need to do unnecessary work. If all available manpower, material 
and funds are concentrated on the region with the highest potentials, work can 
be speeded up and the construction of the project can start at an earlier stage. 
In a former publication dealing with this and other types of soil surveys 
executed with the help of aerial photo interpretation, we introduced the 
system of carrying out the study of soils in the field in carefully selected sample 
areas. In regions with a dense tropical forest or with dense tall grass or reed 
vegetation, this is often rather difficult. It was proved by De Meester in 
Tanganyika (See I.T.C. Publications, series B, 10) that in these cases a some 
what different technique can be followed. Field observations are made along 
lines, that are cut in the vegetation at regular intervals on carefully selected 
sites. A large number of soil profiles are studied along each line. Instead of 
“sample areas”, soils are investigated here in “sample cross-sections”.