3. Procedure:
3.1 Percent reflectance: The data collection consists of first
determining the sun irradiation. This was done by using ground
glass filters on the four sensors of the ERTS Spectro radiometer
and focussing perpendicularly on the sky from top of the ladder.
The procedure is repeated to collect the radiance from water
using plane glass filters and focussing the instrument on the
water.
Radiance and irradiance values were determined between 11 A.M.
and 3 P.M. in order to maintain uniformity of sun angle and
other factors. All readings were taken on clear sunny days only.
3.2 Secchi Depth: Secchi depth measurements were made by immersing
Secchi disc in the water to a depth at which the disc first dis-
appears and the relevant depth noted. The mean depth and maximum
depth of water were measured using a meter stick.
3.3 Extinction Depth: To measure the bottom reflectance effect
the white plate is immersed at various depths and the corresponding
radiance from the plate measured. This was done by two persons,
one standing in the water and holding the plate at various depths
and the other focussing the instrument vertically on the plate.
The percent reflectance of the white plate which is 85 percent
outside the water reduces as the depth is increased and finally
it remains constant. The depth at which the percent reflectance
remains constant is the depth beyond which the water bottom does
not contribute to the measured reflectance of the water body.
A water sample is collected from the site of data collection and
later analysed using a Colorimeter and the relative turbidity of
the water determined.