regarding the reliability of the result and the need of field control when 
using photo interpretation for surveying surficial deposits in well 
wooded areas. This condition becomes still more obvious if one bears 
in mind that owing to cost it hardly can become possible to use air 
photos without shadows. 
A very important stage in all photo interpretation is the use of field 
controls. Photo interpretation can not, mainly for natural reasons, 
totally eliminate field controls. These circumstances will however in no 
way depreciate the importance of photo interpretation. 
For a rational realization of the field control preparations for this 
must be made by the help of stereoscope. When doing so the best 
method is to mark the route on the air photos. To the utmost possible 
extent one eliminates areas which for instance are self-evident from 
the viewpoint of photo interpretation or untraversable. On arrival in 
the field one then has never to hesitate concerning the fieldwork but 
can immediately start on the preexplored route and be assured that the 
result of the fieldwork will be the best possible. 
The field controls are also very important from another point of 
view, namely the interpreters improvement. 
When doing geological maps on the basis of interpreted air photos 
some difficulties appear. Owing to many deficiencies in the available 
topographical maps it is not possible to use them as base maps. Instead 
a totally new base map has to be constructed by help of radial triangula- 
tion. For the transference of the geological symbols from the air photos 
to this map a sketchmaster was used. 
A. photo interpreted survey can quite naturally not contain as detailed 
information as a traditional one. However photo interpretation has 
great advantages, for instance owing to the possibility of looking simul- 
taneously over every point in a vast area one can distinguish facts and 
connections, which only with difficulty can be discovered in the field. 
About the difficulties of photo interpretation they are especially allied 
with the distribution of water and wind transported fine material. This 
is especially valid when sediments, finer than 0,2 mm contain boulders 
and stones, which are not uncommon in the river valleys in northern- 
most Sweden; their origin is not yet clear. 
Ji