
%lesson 4
 

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

%\vskip 0.5cm
\lessons

\centerline{\bf OCEANOGRAPHIC INTERPRETATION OF IMAGES}

\vskip 0.5cm


\centerline{\bf 4 : COMPARISON OF COINCIDENT AVHRR AND CZCS DATA}

%\vskip 1cm

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


To provide experience in the interpretation of multisensor ocean
images comprising visible wavelength data from the Nimbus-7 CZCS
sensor, and infra-red data from the AVHRR sensor; and to explore
the oceanographic applications of the combined dataset.


%\vskip 1cm

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

After completing this unit, students should have learned:-

1) to check the accuracy of geolocation and overlay of data from
two independent satellites and types of sensor,

2) to use the image processing software to compare ocean colour
and sea-surface temperature patterns from different sensors but
the same overpass time,

3) to identify significant differences between the colour and
temperature patterns,

4) to relate these differences to the different physical processes
controlling the two remote sensing methods, and hence

5) to derive further insight into the dynamical or other
oceanographic processes which underlie the image patterns, and
thus gain more information than could be deduced from either
technique on its own.


%\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}

Five image datasets are provided for this lesson.  Four are based
on data from bands 1 to 4 of the Coastal Zone Colour Scanner (the
same as in Lesson 3) converted 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.  The fifth is a sea-surface temperature map derived
using the AVHRR channel 4 on the NOAA-6 satellite (the same as in
lesson 2).  Both were received at The University of Dundee's
Satellite Receiving Station and have been processed using the
Southampton University MIDOCEAN  Software.


Digital Values in the visible waveband images correspond to
radiance in  \break mW/sr/$\hbox{cm}^2$/$\mu$m, multiplied by 25.   {\bf 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.} 

Digital values in the SST image correspond to \degc\  multiplied by
10.  {\bf  Thus the range 0 to 255 on the image corresponds to
temperatures of 0 to 25.5\degc.} 

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}
$$\vbox{\offinterlineskip
 \hrule
 \halign {&\vrule#&\strut \kern0.3mm \  \hfil#\hfil\kern0.3mm \  \cr
 height2pt&\omit&&\multispan3&&\multispan3&&\multispan5&\cr
 &Image&&\multispan3 \hfil Satellite \hfil&& 
 \multispan3 \hfil Overpass data \hfil &&
 \multispan5 \hfil Data Type \hfil&\cr
 &\omit&\multispan{14}\hrulefill&\cr
 &Filename&&Name&&Orbit&&Date &&Time&&Sensor&&Chan&&Product&\cr
\noalign{\hrule}
\burp
&  CC11&&    NIMBUS-7&& 7673       &&  1-5-80 &&   12.00 &&   CZCS &&  1 &&  Radiance(1)&\cr
&  CC12&&      --  &&         &&     -- && --    &&  CZCS&&   2&&   Radiance(1)&\cr
&  CC13&&       -- &&     &&       -- &&  -- &&     CZCS&&   3&&   Radiance(1)&\cr
&  CC14&&       -- &&   &&        -- &&    -- &&    CZCS&&   4&&   Radiance(1)&\cr
 & AC1&&      NOAA-6&& 4388    &&   1-5-80&&   08.00&& AVHRR&&  4&&    BT4 &\cr
\burp}
\hrule}$$


Radiance (1) is water leaving radiance plus aerosol path radiance
(Calculated Rayleigh radiance has been removed.)
BT4 is Sea surface temperature derived from channel 4, without
atmospheric correction  ( i.e surface brightness temperatures).


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

$$\vbox{\offinterlineskip\hrule
\halign{&\vrule#&\strut\quad\hfil#\hfil\quad\cr
 height10pt&   &&\multispan3\hfil  Pixel dimension\hfil && 
   \multispan7\hfil  Image location coordinates\hfil&\cr
&\omit&&\multispan3&&\multispan7\hrulefill&\cr
depth4.5pt& Image&&\multispan3\hfil (minutes of arc)\hfil &&
 \multispan3\hfil Top left\hfil &&
 \multispan3\hfil Bottom right\hfil&\cr
&\omit&&\multispan{11}\hrulefill&\cr
& Filename && Long &&Lat && Long&&Lat&&  Long&&  Lat&\cr
\noalign{\hrule}
height10pt&  CC1x&&   1.0\minutes\ &&     0.5\minutes\ &&     17\degrees 30\minutes W&&
  65\degrees 00\minutes N&&
     8\degrees 58\minutes W&&
62\degrees 52\minutes N&\cr
depth5pt&  AC1&& 1.0\minutes\ &&    0.5\minutes\ &&
    15\degrees 00\minutes W&&
  65\degrees 00\minutes N&&
     6\degrees 28\minutes W&& 
62\degrees 52\minutes N&\cr}
\hrule}$$

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

The approximate location of these images is with Iceland in their
top left hand corner.  The thermal image (AC1) is nominally at the
same latitude as the colour data (CC1) but is shifted 2.5\degrees\   to the
east.  The exact match of one image to the other is one of the
exercises to be performed in the lesson.

Additional files supplied with the image data are palettes
\file{COL1.PAL} and \break\file{TEMP1.PAL} which some users have found appropriate
for visible wavelength radiance and SST representation
respectively, and five look-up-tables (LUTs) \file{CC1x.STR} and
\file{AC1.STR} .

Futher discussion of these images and the oceanographic
environment off Iceland is presented in lessons 2 and 3.


%\vskip 1cm

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

1.  Take a look at all five images provided, and familiarise
yourself with which parts of each image correspond to land, sea
and cloud.

2.   Locate at least one ground control point on images AC1 and
CC1x (the prominent south east corner headland is probably most
suitable) and from its pixel coordinates evaluate its latitude and
longitude according to the information supplied in this lesson. 
Note that you must use an appropriate LUT stretch to mark the
land-sea boundary accurately.  Of the CZCS images, channel 4
should give the best land-sea contrast.  Why? It can be assumed
that all four channels of the CZCS are already co-located.

3.  Compare the predicted location with the true location on the
chart.  How large is the position fixing error.  Establish a
correction factor for each image to enable you to fix the
geographic location of a pixel accurately.  In addition, evaluate
the size of the pixel offset (in rows and columns) between the same
geographical location in images AC1 and CC1x.  This can now enable
you to locate the same position on each image.

4.  Map the location of the Iceland Faroes Front from the
infra-red image (AC1) using squared paper.  Then draw the position
of the high reflectance plume from CC1x on the same map.  Give a
physical explanation of what can be deduced from the juxtaposition
of the plume and the front.

5.  Identify where the temperature and colour signatures do not
appear to be related.  Account for these differences.  

6.  Are there any regions where the colour and thermal patterns
appear to intersect, which could indicate the occurence of
cross-frontal mixing.

[7.  If you have not already done so in lesson 3, locate the
position of the wavelike plumes on the chart, and consider if they
are related in any way to the bathymetry, topography of the
coastline or any other geographic feature.]

8.  With particular reference to the frontal region, list what
oceanographic information is obtainable only by considering both
colour and infra-red image data together.

\vskip 1cm

\hrule

\byline{I S Robinson,   February 1988.}
\vfill
\eject
