The closer union of aerial surveys with highway engineering brought rapid 
advancements in developments and in applications. Much of the progress occurred 
in consequence of planning and conducting to successful completion various types 
of research and development projects in aerial surveys, in highway engineering, 
and jointly in both. Important aspects of the progress were in the development 
of better materials, equipment, and procedures of use; the addition of analyt- 
ical methods; and the employment of printers, measuring and plotting instru- 
ments, computers, and other devices utilizing electronic principles. 
Proper perspective regarding the role of aerial surveys in highway engi- 
neering is not obtainable solely from such historic highlights in development 
and use. In actuality, background theory and basic principles of photogram- 
metry are old but their applications in highway engineering are young. To 
fully understand the relationship of highways to and their ever-increasing 
dependence on aerial surveys, it is essential that we look into the modern 
aspects of highway engineering. 
Highway Service Standards 
By considering current conditions and anticipated future requirements, the 
need for a new highway, or the need for relocation or improvement of an exist- 
ing highway, is determined by engineers in cooperation with planning groups. 
Based on present and estimated future types and numbers of motor vehicles and 
the essentials of service to population, land, industry, commerce and agricul- 
ture, the engineers establish highway systems, designate terminal points for 
specific projects, and define the type of highway to be located, designed, and 
constructed in order to provide the required traffic services. The estimated 
future traffic, usually 20 years hence, by types and number of vehicles, estab- 
lishes the service standards which must be complied with to make each specific 
type of highway effective, in terms of convenience, comfort, and safety, for a 
reasonable service life. 
Among the service standards which are governed by the number and types of 
vehicles, and the design speed at which estimated future traffic may safely 
move on a highway under normal weather and road surface conditions are: Hori- 
zontal alinement, including limits in sharpness of circular curves, desirable 
length of spiral (transition) curves and joining tangents; rates of superele- 
vation; vertical alinement, including desirable and maximum profile gradients, 
and minimum and desirable length of vertical curves; minimum length of horizon- 
tal and vertical sight distances for passing and non-passing; number and width 
of traffic lanes; type and width of median strips where required; width of 
shoulders; limiting rate of slopes in cut, on fill, and for side ditches; desir- 
able and minimum’ width of rights-of-way; full, partial, or no control of access; 
the need for grade separations, interchanges, and frontage roads; and so forth. 
Fully related and applied throughout each successive stage of highway engineer- 
ing and construction, the foregoing standards, and any others that are appli- 
cable, comprise the geometric aspects of the highway. Properly and economi- 
cally fitting the highway and its right-of-way to the topography and land use, 
and adequately preparing detailed construction plans, constitute highway loca- 
tion and design. 
Highway Location in Retrospect 
From the early days of highway construction to less than 20 years ago, 
highway locations, like railroad locations of the past, were usually determined 
by one engineer known as the "locator." The locator walked and/or rode over 
the area between designated terminal points. In effect, as he trudged from 
one visibility vantage point to another, he made a reconnaissance survey by 
on-the-ground examination of the details of topography and land use, and ascer- 
tained their condition and control over positioning of a highway route.