vere availa- 
vay studies. 
We should 
photogram- 
photogram- 
well as in- 
Roads. 
neering; 
tudies. 
to pattern 
nd refining 
the appli- 
engineering 
me predict 
s. Some of 
advance in 
cessary for 
lave a pro- 
well as the 
aid to the 
napping. 
significant 
not widely 
ning. 
vays, Mel- 
"tation are 
losaics and 
  
   
  
   
  
  
  
   
  
   
   
   
  
  
   
    
   
   
   
   
   
  
  
  
   
   
  
  
   
  
  
  
   
  
   
   
  
  
   
   
   
  
  
  
   
  
     
   
(643) 
Planning of new tracks; 
Re-design of existing station yard layouts; 
Improvements to view (sight distance) at existing grade crossings; 
Definition of flood channels and points of stormwater concentration. 
> De 
In order to provide ready means of locality identification, track sleepers or 
ties opposite mile and half-mile posts are painted in code to indicate mileage. 
This work is undertaken just prior to flight. 
Individual photos are rectified and enlarged, and are subsequently assem- 
bled as scale mosaics, the scales usually being 100 feet and 40 feet to one inch. 
Vertical photography is taken on Satefy Aero film with a single lens camera, 
using K-2 minus blue filter and a plate scale of 160 feet to one inch. 
Specification requirements include: 
(1) Photography should be timed to insure definite rail shadow. 
(2) Photographs should not be taken where the track alinement lies within 
20 degrees of the line of the sun’s rays. 
(3) Photographs should be free of cloud effects. 
(4) Photographs should have the usual overlap of 60 percent in direction 
of flight and a transverse lap of 20 percent. 
In those cases where rectification requirements call for suitable field con- 
trol, preliminary survey work is undertaken on the ground prior to actual office 
study of the photographs. In the examination of the effects of flooding, erosion 
damage, and related problems, office study with lens and mirror stereoscopes is 
sometimes supplemented by ground inspection. The flight contractor has recently 
installed a camera with a 25-inch focal length lens to replace the 8'/4 inch pre- 
viously used, resulting in a reduction in rectification requirements. Plan produc- 
tion is accelerated and costs reduced. 
Analysis of 25 replies to the questionnaire indicates a basic similarity in 
the closely related fields of highway and railroad engineering. Stereoscopic exam- 
ination is generally practiced in air photo interpretation for both railroad and 
highway purposes. 
Examples of effective use of photo interpretation. 
Two examples of the value of aerial photography in reconnaissance work 
for highway engineering are cited to illustrate the effectiveness of stereoscopic 
study of small-scale photographs. 
Recently, the U.S. Bureau of Public Roads used aerial photographs entirely 
for a reconnaissance survey between Cortez, Colorado, and Escalante, Utah, an 
airline distance of about 200 ‘miles. This area is, for the most part, arid, badly 
cut up by canyons, and almost inaccessible. Due to the character of the terrain 
and to lack of engineering data, it would have been almost impossible to explore 
the route possibilities by ground methods. Using 1 : 20,000 scale aerial photo- 
graphs, several alternate routes were located at relatively low cost. 
Another instance of a tremendous saving in time, manpower, and money, 
by the use of aerial photographs is the recently completed Mississippi River 
Parkway Survey, jointly undertaken by the National Park Service and the 
Bureau of Public Roads. Six alternate routes, approximating 10.000 miles, were 
located between Lake Itasca, the river's source near the border of Canada, and