CORDIO Status Report 1999.pdf

fade-out of the spectrum. On a “normal” SPOT-image,
where corals can barely be discerned as light-blue
shadows, a type of transformation is applied so that they
appear clearly. The next step is to convert digital data to
reflectance spectra. This is done by correcting for fadeout
in the column of water. Their analysis shows that
reflectance spectra can be distinguished to a depth of 10
metres. They conclude: “these results lend confidence to
the development of procedures that will use satellite or
airborne digital imagery to detect and map coral
ecosystem stress”.
Holden et al. also suggest an area in need of further
research – development of radiative transfer algorithms
and general model development to correct the image for
effects of a water column of varying depth. The problem
is that brightness of the substrate substantially
contributes to the radiation sensed at the sensor. So the
three main variables are: water depth, water quality
(attenuation coefficient) and bottom brightness. If you
know two of these, then simple algorithms are available
for correction of the digital image, but what if you don’t
know the depth for each pixel (which is often the case)
(pers comm).
Image data from SPOT and Landsat are “geocoded”
with aid of the method “Ordnance Survey Maps” and
are corrected radiometrically for sensor calibration, time
of year and atmospheric conditions. The variation in
ocean depth is one of the most cited difficulties with
remote sensing of submarine environments. For example,
the spectral signature of sand at 20 m can be similar
to that of seaweed at 3 m. In order to get around this
problem, Mumby et al. use a special model for correction
for ocean depth. Their objective is not to look at
coral bleaching specifically but, in the first stage, to
distinguish between four types of seabed substrata
(coral, algae, sand and seaweed) and, in the second stage,
to determine the species of the seabed flora. Various
satellite sensors are compared to airborne remote
sensing equipment. Results show that distinguishing
coral from the other three seabed substrata on satellite
images is no problem.
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