(i) Aerial photographs (black and white) 
3. ENGINEERING SOIL SURVEY 
T; 
fte panchromatic (black and white) aerial photographs 
(23cm X 23cm) have continued to gain wide acceptance 
both in India and Nigeria. This is mainly due to 
high resolution, large scale and stereoscopic 
capability which is essential for the interpretation 
of sub-surface calcrete. For engineering and soil 
material survey a scale of Is25,000 has been found 
to be quite satisfactory. 
As the scale is large the land area covered on a 
single photograph is relatively small, the reconnai- 
sance survey of the whole terrain is difficult. 
However a format of large area can still be studied 
by arranging the air photo mosaic. Although the 
system is passive and depends upon solar illumination, 
nevertheless it provides the best source of qualita 
tive data extraction for rapid survey. 
(ii) SLAR imagery (side looking airborne radar) 
It is an active system and independent of solar 
energy. This it is suitable for mapping areas 
obscured by cloud as is the condition prevailing in 
Southern Nigeria. The image strip of small scale 
(1:250,000) currently available in Nigeria, can cover 
large areas on a single photograph, thereby presenting 
a synoptic view of the terrain. But it is devoid of 
stereo coverage which is necessary for the identi 
fication of subsurface calcrete land forms. Though 
general view of the terrain can be studied but 
delineation of soil boundaries for engineering 
interpretation and classification is not practicable 
as the scale is 10 times smaller than the aerial 
photographs. 
(iiia) Landsat imagery (photographic) 
A false colour composite landsat imagery has been 
studied to ascertain the potentiality of landsat 
imagery to the survey of calcrete in N.E. Nigeria. 
The imagery used is a small scale photograph and has 
colour reflections in different bands. The tonal 
variation between calcrete-bearing soil and other 
soil types is a reflection of the colour contrasts, 
which is not quite obvious in this area. Generally, 
the imageries have low colour contrast. The 
colour reflection of sandy soil is light brown with 
little bias towards the calcrete bearing areas, parti 
cularly where vegetation is fairly pronounced and 
along erosional channels or long depression. They 
are only capable of depicting regional linearments 
like the sand dune ridges, but without stereoscopic 
vision. Hence it has poor potential of interpreta 
tion relating to relief and sub surface calcrete. 
Further probe for the location of calcrete has to be 
followed by the interpretation of aerial photographs. 
The lack of stereo image and the corresponding small 
scale are inherent problems associated with the 
interpretation of Landsat imagery. Though it has a 
special advantage of exploiting differences in tone 
signature of terrain objects image in various bands, 
but being passive system offers no solution to the 
acquisition of imagery through the dense cloud cover 
of the southern parts of Nigeria. 
(iiib) Digital landsat imagery 
Digital landsat imageries have been used in developed 
countries. Their processing and classification in 
this way can greatly enhance value of remotely sensed 
data. But the newer digital techniques require heavy 
investment in computing equipment and output devices. 
This is a stumbling block for those working in 
technologically less advanced countries and also for 
users with low budget and lack of duly trained 
technical manpower. Infact, much can be achieved on 
relatively standard equipment so as to avoid adding 
cost of interpretation. 
Soil is the cheapest locally available construction 
material. Hiis can be used either as much, or after 
scientifically processing in various layers of the 
pavement. For any highway project. , reconnaisance 
and detailed soil survey are required to be 
conducted for assessing the suitability of soil as 
sub-grade, sub-base and base course. Information 
about the characteristics of soils can be had 
expeditiously using remote sensing technique, even 
without access to the site. 
3.1 Classification of soils in alluvial plains 
of India 
As a pilot study (Nanda 1969) an area of about 650 
sq. kilometre around Ambala (Haryana) in the 
alluvial plains of north-western part of India had 
been interpreted from the aerial photographs, for 
the identification of various soil types. Some of 
S0JI MAP OF AMBALA AREA INDIA 
(BASfco ON AIR PHOTO INTERPRETATION TECHNIQUE) 
LEGEND 
S M - SILTY SAND 
SC - CLAYEY SAND 
ROAD 
RLY LINE 
MH -CLAYEY SILT 
C L - SILT CLAY 
M L - SANDY SILT 
VILLAGE A TOWN 
RIVER 
s p 5 andy 
FIG 1 
Fig. 2 
Stereopair showing various types of soils in part 
of alluvial plains of India. 
the soil th 
silty sand 
clayey sand 
map thus pr 
Fig. 1 and 
it has been 
interpret 
aerial phot 
interpreted 
3.2 Deline, 
terraii 
Similarly, i 
North-East f 
covering an 
interpreted 
fied accord) 
System. Pr€ 
silty sand ( 
varjf from 11 
whitish tine 
calcareous c 
and met with 
clayey silt 
A stereo pai 
fig. 3 and 4 
4. SURVEY C 
Calcrete is 
aggregates c 
and desert t 
in North-Eas 
called *Kank 
Nigeria. Ge 
surface and 
646