140 Commission VII Invited paper 
Using Airphotos to Identify Construction Sources of Gravel 
by O. W. MINTZER 
The Ohio State University. 
Introduction. 
In this age of increasing highway travel, man has tremendous construction problems. 
In the past ten years pavement design engineers have imposed an unusually high require- 
ment for adequate sources of gravel or other road aggregate. A gravel subbase just 
below the pavement layer is one of the most important elements of the roadway surface; 
thus gravel is a frequently sought-after, natural, non-metallie mineral. Finding adequate 
sources is no menial task, but once a source has been found, transport costs are often 
the difference between a reasonable profit and “breaking even” on a given highway job. 
Hence, it is very important to find the source as near as possible to the site of construc- 
tion. There is a need for a means whereby gravel sources can be found in sufficient 
quantity to meet the higher specification standards for a particular project with a mini- 
mum of transport costs. 
Thirty years ago, when time and construction costs were considerably lower than at 
present, an engineer locating potential sources of gravel could “walk out” the area in the 
vicinity of the site. It is expensive to take the time of an engineer to locate productive 
sites in this old way of “walking it out.” Today the engineer seeks mountain-top-to-valley 
“view” of all the terrain in order to locate the potential sources in a given area. He 
would like a tool which would give him the viewing ability of a giant thousands of feet 
tall. Just such a tool is the airphoto, or view by airplane reconnaissance. In observing a 
pair of photographs the engineer's eyes overlook large areas, just as a giant's eyes 
would, and thus he gets what is known as the areal view or “areal concept.” 
Strange as it may seem, despite the fact that the areal concept is wellknown, the 
airphoto method is not widely used to prospect for gravel sources. 
The importance of locally available granular materials to road and airfield con- 
struction projects often makes it necessary to survey the entire area in great detail in 
the vicinity of the project. Great expense, much time, and extra time correcting mistakes 
are required by the conventional ground reconnaissance methods. Frequently when on 
the ground it is impossible to trace a deposit, because of inaccessibility, thick vegetation, 
or laek of co-operation on the part of a landowner. It is thus costly to “grope in the 
dark" throughout an area. It is time-consuming to make expensive test borings, to get 
mineral landuse options, and to secure the cooperation of landowners. It is also time- 
consuming to attempt to prevent undesirable publicity which could mushroom option 
prices, or even to travel many miles. Thus the use of the areal concept and the aerial 
photographie study looms as a promising tool to reduce field work and time-consuming 
detail investigations to a minimum. 
The areal concept or airphoto method is much the same as the one used by the 
soil scientist when making agricultural soil surveys, by engineers when searching for 
critical soil areas during initial road alignment surveys, or by geologists when studying 
surface geology of a given area to prepare detailed areal geology maps for engineering 
purposes [1-11 incl.]. However, since the airphoto method, as applied to locating gravel 
sources, is not in general use throughout the entire world, it will be the purpose of this 
paper to give a discussion of the method techniques, and applications in the United 
States, Germany, and India. 
Most engineers using the airphoto interpretation method have techniques of their 
own. This author uses the Purdue method that has been outlined in previous publica- 
tions [1]. Perhaps some readers are not acquainted with the method, or the publications;