gi- 
in. 
give 
gi- 
truc- 
en- 
to- 
he 
es 
es. 
“15 
lone 
, may 
information on soil conditions and materials. Other countries, 
England for instance, are well supplied with large scale and 
very detailed topographical maps and detailed geological maps 
are available in nearly all areas. Airphoto-interpretation un- 
der British conditions, therefore, is likely to be useful in 
different ways from those in less developed areas; Reference: 
Dumpleton, and West (39). A more localized examination of the 
possible routes is more appropriate, the emphasis being on loo- 
king for features which will supplement those already shown on 
the topographical and geological maps. Some of the natural fea- 
tures sought, are areas of poor drainage, marshy areas, land- 
slides and unstable ground, mining subsidence, swallow holes, and 
rock outcrops. Man-made features of interest include any changes 
Since the last map revision, e.g. new roads or buildings, demo- 
lition of buildings, extensions to quarries and gravel-pits, re- 
moval or changes in field boundaries, pipelines and movement of 
mining spoil heaps. 
The application of airphotointerpretation relate mainly to the 
route location and preliminary stages of Highway Design. 
Investigation of possible alignments of different fundamental 
centerlines can be made photogrammetrically. The Balplex Plotter 
(see fig 2) may be mentioned as an instrument suitable for DÜIO- 
ducing an efficient and convenient overall view of the entire 
photogrammetric model or set of models including preliminary 
measurements of the loncitudinal profiles along the fundaman- 
tal alternative centerlines. 
The question of the requirements on geometrical accuracy in the 
terrain data is discussed below (see page 15). As it is a ques- 
tion of only preliminary information, complete horizontal and 
vertical control for the different photogrammetric models is 
generally not required. An arbitrarily located baseline, measu- 
red in the field can be satisfactory for the scaling of the model. 
Vertical control can be achieved by simple measurements, baro- 
metric etc, if it is not already available. 
The control points for the evaluation of the models can also be 
achieved by aerial triangulation. This method is mainly used