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SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962 
Introduction 
During recent years the use of photo interpretation in route location has 
expanded rapidly. This paper discusses briefly the principles and practice of 
this technique based on experience gained from various field projects. The 
discussion includes: a. technique of photo interpretation; b. integration of pho 
to interpretation in the route location program; c. The Quebec Cartier Mining 
Railway in Canada as an example; d. specific conditions affecting route 
location evaluated by photo interpretation; and e. concluding remarks. 
Technique of photo interpretation 
The technique of photo interpretation employed in route location studies is 
generally similar to that used in other engineering projects but necessarily 
oriented to its specialized ultimate objectives. It usually calls for a study of 
relatively large areas, with emphasis on surface soils and geology. The normal 
procedures are: a review of existing information pertinent to the area, the study 
of photo images in three dimensions and interpretation of these images from 
the standpoint of their engineering significance, the planning and execution of 
field investigations and finally the mapping and presentation of the inter 
pretation. The task is usually performed by an engineer with adequate geol 
ogical background or an engineering geologist with a working knowledge of 
soils. 
Integration of photo interpretation in the route location program 
Route location is usually accomplished in three stages: reconnaissance, 
preliminary location and final location. Each stage refines the location to a 
higher degree than the preceeding stage. The basic task in location is the fitting 
of the road or railroad structure to the natural features of the area in a manner 
that will be most economical in construction and operation. This demands that 
natural features must be effectively inventoried or assessed. 
The natural features to be assessed in a location study are: topography, 
drainage, property values, soils and geology, borrow sources, vegetation and 
special trouble areas. Imposed upon this information about the natural features 
of the area is the engineer’s knowledge and experience to determine the length, 
grade and alignment of the route and the type of structures. 
Reconnaissance is the activity of selecting one or more corridors between 
the terminals which appear to contain the feasible locations. In current prac 
tice, these corridors must be designated with sufficient accuracy to allow 
economical coverage by mapping photography which will yield 2-, 5- or 10- 
foot contours (primarily) depending upon the topography). These maps will 
provide the basis for preliminary location. 
Photo interpretation study in the reconnaissance stage covers a wide area 
between the terminals to be assessed at a small scale. In areas for which topo 
graphic maps are not available, topography can be evaluated with varying