
%lesson3



%\centerline{
%\underbar{Practical Lessons in Satellite Oceanography}}

%\vskip 0.5cm
\lessons 

\centerline{\bf OCEANOGRAPHIC INTERPRETATION OF IMAGES}

\vskip 0.5cm


\centerline{\bf 3 : COASTAL ZONE COLOUR SCANNER DATA OFF ICELAND} 

%\vskip 1cm

%\centerline{\bf \underbar{Aim of Lesson}}   
\aims

To provide experience in the interpretation of multichannel ocean
colour images based on visible wavelength data from the Nimbus-7
CZCS sensor, and to demonstrate some of the oceanographic
applications  of such data, in a coastal region and in the
vicinity of an Oceanic Front.


%\vskip 1cm

%\centerline{\bf \underbar{Specific Objectives}   }
\objectives

After completing this unit, students should have learned:-

1) to use the image processing toolkit in order to enhance the
spatial patterns in visible wavelength images of water-leaving
radiance,

2) to use the image proccessing software to extract colour
information from the four CZCS channels,

3) to define the spectral signature of water-leaving radiance at a
particular geographical location,

4) to suggest interpretations of what in the water is contributing
to the patterns on the CZCS images,

5) to interpret the ocean colour as a tracer of dynamical
processes in the ocean, and

6) to measure the length scale of dynamical features.


%\vskip 1cm

%\centerline{\bf \underbar{Background Information}}   
\background

{\bf The Bilko image-processing software}


No prior knowledge of the BILKO system is required.  The online
 HELP  facility, accessed by typing  \type{H}, explains the operation of
the current command and  \key{escape}   returns to the main menu. This
lesson will illustrate the application of some of the facilities
provided by the toolkit, and guidance will be given in using them
to best effect.


 {\bf Other equipment} 

In order to work through this exercise, you will probably find it
helpful to have a calculator, graph paper for plots, and coloured
pencils if you want to sketch some of the features observed.  To
relate the images to the underlying oceanographic processes, it
will be useful to refer to a hydrographic chart covering the
region under study.


 {\bf The images used for this lesson }

Four image datasets are provided for this lesson.  They are based
on data from bands 1 to 4 of the Coastal Zone Colour Scanner
received at The University of Dundee's Satellite Receiving Station
from a single overpass of the Nimbus-7 satellite.  These data have
been processed using the Southampton University MIDOCEAN 
Software to convert the raw data into resampled, geolocated images
of water-leaving radiance.  A correction has been applied to
remove the atmospheric path radiance contributed by Rayleigh
(molecular) scattering of light into the sensor's field of view,
but no correction has been made for the effect of aerosol
scattering.
   
{\bf  Digital Values correspond to radiance in mW/sr/$\hbox{cm}^2$/$\mu$m,
multiplied by 25.  Thus the range of 0 to 255 available on the
given image datasets corresponds to 0 to 10.2 mW/sr/$\hbox{cm}^2$/$\mu$m.} 
In this lesson no use will be made of the actual radiance values,
but potentialy such information can be exploited in quantitative
calibrations in terms of water-quality parameters.

The geolocation is expected to be accurate to within 4 km.  The
images have been resampled onto a Latitude/Longitude grid with
cell size 0.5\minutes\   in latitude and 1\minutes\   in longitude. It should be
recalled that at latitude 60\degrees N a pixel which is 0.5\minutes\ high in
latitude and 1.0\minutes\ longitude wide represents a square of side
0.93km.   This resolution is justified by the sensor viewing at
nadir where it has a resolution of 825m, although at scanline
extremities such a high resampling resolution is not justified,
especially in the along-scan direction (approximately east-west).

The image definitions are as follows:-
\def\burp{
height2pt&\omit&&\omit&&\omit&&\omit&&\omit&&\omit&&\omit&&\omit&\cr}
$$\vbox{\offinterlineskip
 \hrule
 \halign {&\vrule#&\strut \kern0.3mm \  \hfil#\hfil\kern0.3mm \  \cr
 height2pt&\omit&&\multispan3&&\multispan3&&\multispan5&\cr
 &Image&&\multispan3 \hfil Satellite \hfil&& 
 \multispan3 \hfil Overpass data \hfil &&
 \multispan5 \hfil Data Type \hfil&\cr
 &\omit&\multispan{14}\hrulefill&\cr
 &Filename&&Name&&Orbit&&Date &&Time&&Sensor&&Chan&&Product&\cr
\noalign{\hrule}
\burp
&  CC11&& NIMBUS-7&&7673  &&  1-5-80&&    12.00&&    CZCS&&   1&&   Radiance(1)&\cr
&  CC12&&         &&    &&      &&          &&   CZCS&&   2&&   Radiance(1)&\cr
&  CC13&&         &&      &&    &&          &&   CZCS&&   3&&   Radiance(1)&\cr
 & CC14&&        &&         &&  &&          &&   CZCS&&   4&&   Radiance(1)&\cr
\burp}
\hrule}$$


MIDOCEAN software product Radiance(1) is water leaving radiance plus aerosol path radiance (Calculated
Rayleigh path radiance has been removed).

The geometric resampling and location coordinates of the images
are:-

$$\vbox{\offinterlineskip\hrule
\halign{&\vrule#&\strut\quad\hfil#\hfil\quad\cr
 height10pt&   &&\multispan3\hfil  Pixel dimension\hfil && 
   \multispan7\hfil  Image location coordinates\hfil&\cr
&\omit&&\multispan3&&\multispan7\hrulefill&\cr
depth4.5pt& Image&&\multispan3\hfil (minutes of arc)\hfil &&
 \multispan3\hfil Top left\hfil &&
 \multispan3\hfil Bottom right\hfil&\cr
&\omit&&\multispan{11}\hrulefill&\cr
& Filename && Long &&Lat && Long&&Lat&&  Long&&  Lat&\cr
\noalign{\hrule}
height10pt& CC1x&&   1.0\minutes\  &&   0.5\minutes\  &&      17\degrees 30\minutes W&&
  65\degrees 00\minutes N&&
     8\degrees 58\minutes W&&
62\degrees 52\minutes N&\cr}
\hrule}$$

[Where reference is made to all four channels the terminology CC1x
will be used to mean CC11, CC12, CC13 and CC14]

These details can be checked in the CC1x.HDR files by using the
TYPE command in MS-DOS.  The approximate location of the images is
with South East Iceland in their top left corner.

Additional files supplied with the image data are a palette
 \file{COL1.PAL} which some users have found appropriate for visible
wavelength radiance representation, and four look-up-tables (LUTs)
 \file{CC1x.STR} .


{\bf Factors affecting the interpretation of Ocean Colour data} 

Ocean colour as viewed from space is controlled by the optically
active constituents of the sea water.  Pure sea water appears blue
because water absorbs red light and scatters blue. If suspended
sediment is present there is a tendency for all colours of light
to be reflected.  If there is a population of live phytoplankton,
the chlorophyll absorbs preferentially in the blue so that the
water appears greener.  Dissolved organic compounds from decaying
vegetation (so called yellow-substance), absorbs blue and green
light.

The coastal zone colour scanner was designed to detect some of
these factors.  Its spectral channels are:-
$$\vbox{\offinterlineskip\hrule
\halign{\vrule#&
\strut\quad #\quad &
\vrule#&
\quad #\quad &
\vrule#&
\quad#\hfil\quad&
\vrule#\cr
height11pt&1&&   433-453nm &&(blue)&\cr
&2&&  510-530nm& &(green/blue)&\cr
&3&&   540-560nm &&(green)&\cr
depth6pt&4& & 660-680nm& &(red)&\cr
}\hrule}$$
Relative variations of radiance in the different channels can
provide clues about what may be in the water column.  If channel 1
becomes dark but channel 3 does not, then it is likely that there
is an increase in chlorophyll.  Although the data should be
completely atmospherically corrected before a quantitative
chlorophyll algorithm can be applied, examination of the
green/blue ratio (channel 3 / channel 1) can indicate whether a
plankton bloom is present.  If the radiance increases across the
whole spectrum, then it is more likely that increased suspended
sediment in the near-surface layer is the cause.  Sometimes the
particulate material causing the scattering may be blooms of
phytoplankton with highly reflective scales (e.g. coccolithophores). 
If the blue channel increases in preference to the others, then it
may be the effect of reflection from shallow bathymetry (down to
20m depth in clear water), since the shorter the wavelength the
further the light penetrates.  These general guidelines, along
with some reasonable assumptions about local oceanographic
conditions, should form the basis of interpretation of the colour
signatures on the images presented in this lesson.


{\bf Relevant aspects of the local oceanography }

Around the coast of Iceland, the \lq water quality\rq\ parameters may be
expected to be strongly influenced by local runoff from the land,
and by interaction with the shore.  East of Iceland, at about 65\degrees N, the western end of the Iceland-Faroes Front is found, which
forms a boundary between the surface wind-driven Sub-arctic Gyre
to the south and the Arctic Ocean to the north. 

The waters at these high latitudes are not strongly stratified
except at the frontal zone.  In May only a weak thermocline will
have developed, and therefore near surface tracers should reveal
something of the dynamical structure of the upper ocean as a
whole.  Close to the shore the mixing in the shallow water should
lead to fairly uniform conditions in the vertical. 

%\vskip 1cm

%\centerline{\bf OUTLINE of LESSON }
\outline



The lesson is likely to take at least an hour.
 
 1.   Enter the Bilko software, load image CC13 and display. 
It is often good practice to view channel 3 of CZCS first since if
there is any reflective signal it tends to show up most clearly in
this channel.  It and channel 4 also normally provide a better
contrast between land and sea than channels 1 and 2.

 2.   If desired use function \menu{Modify palette} to produce a
colour scale to your own taste which seems more representative of
increased radiance than that provided as default on entering Bilko. 
Alternatively use the function \menu{Load palette} to enter a palette
provided in file \file{COL1.PAL} . 

 3.   Use function \menu{Modify LUT} to improve the image until it
reveals patterns of radiance structure in the water (if there is
any).  Note that if a very regular lined structure starts to
appear, it is probable that the digitisation limits of the sensor
have been reached, and further stretching may tend to obscure the
oceanographic features which are being sought.

Identify on the image the areas which have higher or lower
radiances than the main peak of the histogram, and account for
these differences.  Which is land and which is cloud?

 4.   Repeat stage 1-3 for the other images CC11, CC12 and CC14. 
In each case sketch a chart of the patterns of variability which
are displayed.  

 5.   Compare and contrast the images for the different channels. 
Use the \menu{Cursor} routine to obtain spectral signatures (i.e. the
values of radiance in all four channels) at selected points in the
scene which you consider to be representative of the different
regions and features.  Bearing in mind the possible local
oceanographic factors, and also the general guidelines given above
about interpreting colour signatures, suggest what it is in the
water which is producing the observed colour signatures.

 6.   Use the two-band ratio function in the \menu{Multi-image} commands
to obtain a ratio of bands 3 and 1.  Because a full atmospheric
correction has not been applied, this will not be a true estimate
of the water-leaving green/blue ratio, but it may reveal if there
are any underlying patterns due to chlorophyll absorption rather
than the broad-band reflection processes which appear to be
dominant in this case.  [Some experimentation with the ratio-ing is
recommended.  The procedure automatically \lq stretches\rq\  the results
linearly to fit the minimum and maximum values of the ratio (found
within the prescribed box) to the numerical range 0 to 255.  Thus
if the zero is to be true, the prescribed box must reach down to
the x axis]

 7.  Measure the length scales of the patterns evident on the
images.  The \menu{gradients} function may help to identify where the
features are strongest.  Is there any evidence that different
features are being revealed in different channels?

 8.   Assuming the water parameter interpretation considered in 5,
account for its spatial distribution in terms of the regional
processes, e.g. bathymetry, coastal discharge, fronts, waves,
circulation etc. Reference to a bathymetric chart may be helpful
at this stage.  Can the ocean colour remote sensing tell us
anything we could not otherwise easily find out about the
oceanography of the region?

\vskip 1cm
\hrule

\byline{I S Robinson,  January 1988.}


\vfill
\eject

