terme TES 
  
  
ESRI 
TESTS EEE 
(648) 
as to the permanency and regimen of the water bodies. In arid regions, recogni- 
tion on air photos of plants that serve as indicators of shallow ground water can 
direct the search on the ground. The salinity of surface water or shallow ground 
water may also be indicated on the photos by characteristic plants. 
Weather conditions affect suitability of a site in regard to both ease of con- 
struction and desirability for operation. Air photos can add little to detailed 
information on weather and climate, better available from other sources. Some 
general weather indications, however, can be interpreted, mostly pertaining to 
the direction of winds, an important consideration for orienting runways. To- 
pography commonly affects the direction of the lower winds and may create local 
wind disturbances. For example, undesirable updrafts or downdrafs may be 
expected along an escarpment. The location of a site near the edges of a terrace 
or plateau or on a small segment of terrace or plateau surface bordered by es- 
carpments, therefore, should be made with careful investigation of air currents. 
The direction of prevailing winds may be indicated by the shape of active sand 
dunes. Snowdrifts may give some notion of direction of wind and kind of pro- 
tection needed against drifting. The vegetation type in an area indicates the 
general climate — length of growing season and the rainfall and temperature 
during the growing season. Shapes of isolated trees may indicate wind exposure 
and wind direction. In arctic regions, thicker vegetative growth indicates snow- 
collection areas. 
Access determines the feasibility of reaching the site, first for the ground 
survey, then with equipment and materials, and finally for operating the com- 
pleted airfield installation. Air photos can be used for tracing the existing trans- 
portation routes and favorable terrain (in regard to slope, soils, drainage, vege- 
tation, and other obstacles) for cross-country movement or road and railroad 
construction. Where the site is far removed from transportation terminals, air 
photos provide a means for rapidly scanning many miles of intervening country 
and appraising distance against ease of movement and construction. Access may 
involve the selection of sites for coastal landing and construction of port faci- 
lities. This, as well as the evaluation of road alignments, presents engineering 
problems other than those of strictly engineering. Air-photo interpretation is 
applicable to these problems as well but will not be discussed further here. 
Conclusion. 
Aerial photos provide some unique services to airfield engineering and are 
an important aid to ground investigations. They are applicable, in some degree, 
to the appraisal of almost all the factors that determine the suitability of a site. 
When necessary, preliminary site selections and estimates of anticipated engi- 
neering problems have been made by the use of air photos with only a general 
knowledge of the region and little or no ground investigation. However, a reli- 
able and detailed appraisal requires air-photo interpreters who have engineering 
knowledge and scientific training in fields pertaining to the natural features to 
be interpreted, including geology, soils, and botany, particularly in specialized 
fields of forestry and ecology. A familiarity with the region of analogous regions 
and any available pertinent maps and reports greatly increase the accuracy of 
interpretation. The nature of the terrain itself has a bearing on the amount of 
reliability of interpretation possible. Some terrain lends itself to more complete 
   
    
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
ana 
surf 
for 
pro 
pro 
the 
spre