GVII-XO (Sup *) 
reasons. 
11® When one or two alternatives have crystallised in 
this way* they will be if photography season permits, 
photographed from lower altitude in order to secure a 
better photo interpretation and an accurate survey. 
When determining what height accuracy is necessary* 
it should be observed that it is not the absolute height 
in meter over sea level but the relative height in re 
spect to the joining part of the line which is essential. 
The report from Switzerland states that the enter 
prises occupied with the planning and the execution of 
our large hydroelectric power development programs in 
that country also make extensive use of photographs® 
They benefit thereby in all and everyone of their many- 
sided activities* such as construction of dams and 
reservoirs, outlay of communications* pipe and power 
transmission lines* etc® As problems of special impor 
tance, which can hardly or not at all be attacked and 
solved satisfactorily and economically without the use 
of photographs* may be cited the study of distribution* 
configuration and behavior of detrital formations and 
the accurate determination as well as the control of 
areas threatened by landslides* etc. 
Road and railway locations also study photographs 
with great advantage in the planning as well as during 
construction periods. In mountaineous regions* first 
and foremost* photographs provide indispensable informa 
tions* e.g. with respect to the definition of potential 
danger zones* avalanches* slides* rockfalls, and the 
protective measures to be taken. 
City Planning Applications 
For city and regional planning in Switzerland* 
photographs are employed with increasing success« This 
is mainly due to the fact that* in contrast to maps and 
plans* photographs allow an easy* accurate and economic 
way of establishing and controlling the state of more or 
less rapidly changing conditions, as they regularly occur 
in densely populated or intensely worked areas. Examination 
under the stereoscope is deemed essential. It is in this 
branch of civil-engineering that mosaics are more frequently 
employed than elsewhere. They serve mainly as easily legible 
instruction and demonstration material or as illustrations 
in the discussion of projects.