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fore determined by differentiating the value of the various elements. Here the knowledge 
and experience of the analyst comes to the fore. 
The most important advantages of surveys carried out by means of photo-analysis, 
besides a considerable saving in time, personnel and money, are: 
1) that the method is suitable for making semi-detailed maps (scale 1:50.000) ; 
2) that before any field work is carried out, the whole area is thoroughly studied; 
3) that a general idea is obtained as to which areas are more and which are less important 
for the purpose of the survey; 
4) that during the fieldwork the surveyor can concentrate on those areas which are 
doubtful in the analysis; 
5) that an idea is obtained as to where soil profiles in quarries, roadsides, ete. can be 
easily studied. A route map may be composed beforehand; 
6) that more time can be devoted to special studies of the terrain, profile descriptions, 
simple field tests, samplings, etc. 
7) that the maps produced are more accurate in the course and situation of the boundaries. 
This specially holds good for small-scale surveys. 
8) that the method is reproduceable; 
9) that after the completion of the field work and the laboratory research, certain problems 
can again be studied on the photographs. 
Interpolation and extrapolation. 
The interpolation method necessitates such a choice of the key areas that the terrain 
conditions in the intermediate areas as far as this is controlable from the photo-image 
  
-— are of the same type as in the key areas. Correlations observed in the terrain between 
phenomena on the photo-image and soil condition can only be interpolated on this condition. 
In some cases it is therefore preferable to select the key areas in such a way that they 
comprise also the transitional zones. Interpolation can then be carried out to one side as 
well as to the other side. Consequently, the key areas will not be situated at equal distances 
nor be of the same size. 
The photo-analysis for this purpose may be carried out either before or after the execu- 
tion of the terrain work. The first way is the most advantageous one. The whole terrain is 
studied thoroughly before field work starts and the key areas can be determined very effi- 
ciently. Immediately after the terrain work in the key areas is finished, certain doubtful 
points in the analysis of the intermediate areas can be checked. The execution of the analysis 
after the work in the key areas has the advantage that the analysis can be carried out with 
more immediate success. Yet, the whole area must be studied beforehand in a lay-out in order 
to select the key areas as precisely as possible. Moreover, this working method has the 
disadvantage that after the analysis has been completed, the terrain must again be visited 
for a final control. 
The extrapolation method is based on a limited number of field data collected during 
a reconnaissance trip and for the rest on direct photo-interpretation. Photo-analytical maps 
are made as guides for the reconnaissance tour. In this way the traversing of the terrain 
yields more accurate and better maps. 
Different methods may be followed. Either first a small-scale photo-analytical map to 
check in the field is made of the whole area or the photo-analysis is started with some well- 
selected parts representative for larger areas. After field control of these more detailed 
photo-analytieal maps and establishing the definite mapping units a final analysis is made 
for the whole of the terrain. The second method had many advantages; it has only this 
disadvantage that special points of importance to the survey outside the selected areas may 
be missed. A second visit to the terrain may be necessary. 
In some cases it will have sense to make maps via the photo-analysis without any 
field control, only relying on data from literature and the experience and knowledge of the