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(646) 
must be planned in areas that commonly are not available for ground investiga- 
tion. During World War II the German Forschungsstaffel used air-photo inter- 
pretation extensively for all types of military intelligence including airfield 
siting (reference 5). 
To the investigator on the ground at the site, air photos provide a means 
of organizing the observable details in relation to each other and to the general 
setting. Unit areas of apparently similar soil, vegetation, or other terrain con- 
ditions can be outlined as an aid to laying out a pattern of soil sampling and 
other ground tests. The air photo is a convenient base for mapping. As data are 
plotted, relationships can be established between what can be seen on the photo 
and what exists on the ground, making possible extrapolation of results beyond 
the spot tests. Some phenomena, such as soil polygons, rock structures, and lines 
of subsurface drainage, are more clearly seen from the air than from the ground. 
Without the use of air photos, some unfavorable conditions may be overlooked 
until construction difficulties call attention to them. 
On occasion, air photos may be useful in solving maintenance problems. 
Sink holes have developed under some runways built on reef-limestone terrain 
of the Pacific islands. In some of these places the location of sinks is associated 
with lines of subsurface drainage, in others with old strand lines. Both conditions 
were traceable on air photos?'). Localizing such incipient danger points permits 
preventive maintenance. If the danger had been recognized earlier, air photos 
could have been used for laying out runway alignments that would avoid the 
bad spots. 
Interpretations possible. 
Aside from external artificial conditions, such as real estate values and 
traffic requirements, which may be the sole determinant for locating an airfield, 
the selection of the naturally best-suited site within an area depends on these 
factors: 
1. Grades and alignments 6. Construction materials 
2. Approaches 7. Water supply 
3. Subgrades and foundations 8. Weather 
4. Drainage 9. Access, 
5. Clearing 
An accurate appraisal of these factors and a careful selection of the site 
will save time and materials during construction and may prevent costly 
maintenance. Air-photo reading and interpretation can be applied in some 
degree to all these listed factors. Some factors can be directly identified or 
measured, others must be interpreted on the basis of geologic, pedologic, hydro- 
logic, and ecologic relationships. 
Grades and alignments are determined by the ground slopes and the dimen- 
sion and orientation of the flatter or more suitable parts of the terrain. Photo- 
grammetric means can be used for measuring slopes. A knowledge of landforms 
can provide a rough estimate of expected slopes and suggest details of micro- 
relief that may not be evident on the photos, such as the rough conditions on 
surfaces of lava flows. Approximate estimates of the amount of rock and soil 
1) Personal communication, Delos E. Flint, U.S. Geological Survey. 
  
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