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to the nornal inverse relationship, in the light of low turbidity
of water, indicated how the colour of polluted waters can affect
the backscatter /éharacteristics due to decreasing absorption, and
hence attenuation, with decreasing colour indicated by increasing
secchi depth (Fig.4). Though the non-uniform vertical distribu-
tion indicated by quantaspectrometer measurements called for depth
sampling practical field constraints limited water sampling to
surface strata.
3. AERIAL SUHVZYS
3.1 Mission planning and execution
" Aerial gurveys, using metric camera and modular multispectral
11 band scanner systems mounted on a Dakota DC-3 aircraft, were
conducted over the study areas in April 1931 at the time of maxi-
mun pollution conditions. Black-and-White panchromatic and colour
infrared photography was obtained along with digital scanner data
through suitable flight planning. To account for the spectral de-
gradation away from the nadir of the scan the line spacing was
adjusted considering only the central 45° scan angle swath as con-
taining information suitable for analysis. To account for the
differing ground coverage of the RMK 30/23 camera and scanner sys-
tems, the former was obtained at 10,000 ft. above ground level
(ACL) while the latter was obtained at 3000 ft.AGL, Flying was
restricted to non-noon hours to avoid specular reflection, atleast
over the centre portion of the sean. The sequence of flying over
different flight lines was carefully specified to enable acquisi-
tion of simultaneous groundtruth along these lines. The mission
was executed on 14th April 1981 over Godavari river area and on
9th and 21st April 1981 over Husainsagar area. The first flight
Over Husainsagar on 9th April was incomplete since aerial photo-
craphy could not be completed and so was again attempted on 21st
ADTII194.
3.2 Groundtruth collection
Collection of water samples simultaneous with aircraft over-
pass was undertaken in the Godavari river area. Five boats vith
scientific personnel on bosrd were employed in this effort. Along
vith water sample collection, measurements were also made with re-
gard to secchi disc depth, total depth and temperature. The
flight plan was so arranged that the time between coverage of suc-
cessive flight lines was adequate for the 5 boats to move over,
occupy their pre-specified positions in the new line, and be ready
for groundtruth when the plane came overhead on that linc. The
groundtruth effort was also suitably designed such that more sam-
ples were collected in the main pollution dispersal zone. Totally
35 samples were collected and analysed for parameters such as pH,
specific conductance, colour, turbidity, total :dissolved and sus-
pended solids, chlorides, sulphates, phosphates, nitrites and
nitrates.
A time lag between aircraft overpass end water sample collec-
tion had to be allowed in Husainsagar as a result of some con-
straints with regard to non-availability of boat, movement of
water weeds and also since the relatively unchanging water qua-
lity permitted this time lag. While the non-availability of the
613
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