il 
Furthermore, aerial surveys make it possible for everyone involved to reach a 
better understanding of perplexing questions, and the proposed solutions and 
reasoning involved. They aid users to prepare and preserve a composite illus- 
tration of their completed engineering, and reviewers to evaluate quality and 
sufficiency of the engineering work accomplished. 
APPLICATIONS OF AERIAL SURVEYS IN HIGHWAY ENGINEERING 
The two principal categories into which highway engineering are divided, 
with respect to building roads, are preliminary engineering and construction 
engineering. The specific nature of work to be performed gives rise to the 
maxim that those who should use aerial surveys in the highway engineering 
stages of each category are the particular specialists who have the responsi- 
bility of accomplishing the work. To point to the diversification that exists 
from stage to stage, to indicate the sequence and continuity within each stage, 
and to show that results of aerial surveys and associated engineering work 
accomplished in the preceding stage of each category serve as detailed guides 
for work in each subsequent stage, a brief resumé of the engineering work accom- 
plished is given at the beginning of each stage before presenting applications 
of aerial surveys in that stage. 
Preliminary engineering begins with certain aspects of planning and ends 
with the location survey, after which construction engineering begins. 
APPLICATIONS IN HIGHWAY PLANNING 
Highway systems are established by class according to the traffic and land 
use centers to be served. Within the specific systems, terminal points for 
highway surveys and design, preparatory to construction by projects, are des- 
ignated and priority of construction is established. The highway service stand- 
ards are decided, after evaluating traffic and land use, on the basis of types 
and intensity for the present and future, usually 20 years hence. 
Utilizations of aerial surveys in this planning stage are not detailed in 
character. This is because overall aspects and general relationships, partic- 
ularly with respect to land use and topography, are the principal considera- 
tions. Aerial surveys, therefore, are used to determine existing land use by 
type and intensity, in both rural and urban areas. This use involves classi- 
fication and determination of the scope of the various types of land use that 
affect planning and determination of highway systems, principally the centers 
of population, commerce, and industry and intervening farming, grazing, recre- 
ational, and undeveloped areas. Interrelated with the foregoing is the need 
for estimating the possible changes in land use based on the study of patterns 
of past changes in the rural and urban areas, as obtained from serial photo- 
graphs of successive and adequately separated periods of time. 
Aerial surveys are used in the planning stage for correlation of traffic 
origin and destination surveys with existing land use and anticipated or fore- 
casted uses of land as they may be changed or intensified by traffic corridors. 
Aerial surveys are also utilized to study traffic conditions through the obtain- 
ment of aerial photographs of congested traffic, traffic bottlenecks, and free- 
moving traffic. On the photographic mosaics and on oblique photographs, con- 
ditions on existing traffic corridors and proposed new traffic corridors are 
illustrated. 
Planning is a continuing process, but completion in detail for a specific 
segment of any highway system sets the governing standards and guidance pat- 
terns for the four location survey stages of preliminary engineering to follow.