l-
e
\
A
1
|
» x
1 “
S
]
5
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