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SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962 
agricultural development. In some countries this work has already been done. 
However, there are still many countries where potentials are unknown. 
The easiest and most efficient way to select promising areas, is to eliminate 
the areas which are not promising at all. Various factors limiting future devel 
opment, such as rough topography, dense vegetation, location, geology, soils 
and in arid regions also water, can be studied from aerial photographs and 
mosaics. Without any study in the terrain itself, large parts of a country can 
often be eliminated for further investigations. The areas not eliminated are 
often less than 10% of the total area, and are still too large. Therefore a 
further selection has to be made for these areas. 
The work to be done in this phase of the investigations is rather simple. 
Photographs, and in particular mosaics are studied, and a list of limiting 
factors is set up. The whole country is classified in regions, and each region is 
delineated roughly on mosaics or maps, and characterised by some symbols, 
indicating the limiting factors. Other information, e.g. on climate, geology, 
can also be taken into consideration. 
An important point is to know the minimum size of the regions to be indi 
cated on the final map of the whole country. Major potential agricultural 
regions suitable for a development project should have a minimum size of 
about 10,000 to 25,000 ha. In such a case, all smaller areas can be neglected, 
and aerial photo interpretation work can be simplified during the first phase. 
This has also a great influence on the scale of the aerial photographs, and 
consequently on the aerial survey. Small scale photography (for example 
1 : 50,000) for the whole country will be sufficient from the point of view of 
selecting large potential agricultural regions. 
It is worthwhile preparing a small scale exploratory map e.g. 1 : 1,000,000 
of the whole country, indicating the various general physiographic regions, 
their limitations and advantages for development. If it is possible to carry out 
some field investigations, the promising regions should be visited and some 
general information on vegetation, geology, water and soils should be collected. 
The results of the investigations of the first phase should be described in a short 
report, and handed to the reponsible authorities, who will decide which of the 
regions shall be selected for further investigation in the second phase of develop 
ment. 
Soil investigations in the second phase 
The purpose of the investigations in the second phase is to study the potentials 
for agricultural development of one, two or three regions, which after the 
first phase seem to be promising. The area is now limited in size in comparison 
to the area studied in the first phase which covered the whole country. Often 
no more than 100,000 to 500,000 ha have to be investigated for one project. 
The main point now is to study the terrain and soil conditions, in such a way 
that the Government can decide whether to start with a development project 
or not. Sometimes, when more than one region has to be investigated, the