INTERPRETATION OF MICROWAVE REMOTE SENSING IMAGES EXERCIS

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INTERPRETATION OF MICROWAVE REMOTE SENSING
IMAGES EXERCISE
dr.ir. Jan Clevers Centre for Geo-Information Dep
t. Environmental Sciences Wageningen UR
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Assignment Study the following sheets in small
groups and try to answer the questions
raised.The objective of the exercise is to look
at variuos types of remote sensing images and to
gain a first impression of differences and
similarities.With some of the images we will
look more at the geometric aspects, with other
images more at the thematic aspects.First
answers for each sheet the question before moving
on to the next one.In case things are not clear
or when there are questions you may contact one
of the supervisors.
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Question 1 Here you see two airborne images. One
is an optical image, the other is a radar image.
Which one is the radar image (left or
right)?What do you think the circles are?
? Wageningen UR 2002
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Here you see a picture of a so-called central
pivot irrigation system. This is e.g. commonly
applied in the western United States en it causes
the circles which may be observed from an
aircraft. Water is pumped to the centre of the
circle and a large broom rotates around this
supply point.
? Wageningen UR 2002
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Question 2 Here you see a detail of the previous
two images.Do you have any idea what the stripes
at A are and what causes them to appear on the
image (remember that a radar looks sideways, in
this case from the left)?Do you have any idea
what the stripe at B is?What do you think the
stripes at C are?
? Wageningen UR 2002
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Question 3 It is known that sea waves near the
coast can provide information on sea bottom
topography, when conditions are suitable
(moderate wind and strong tidal currents). As a
result radar images can be used for a so-called
Bathymetry Assessment System (BAS).Below we see
an image from the ERS-satellite of part of the
province Zeeland in The Netherlands.Why shows
the water darker tones when going more inland?
? Wageningen UR 2002
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Question 4 Below we see two images of the San
Francisco area.In radar, some features have
tonal signatures quite unlike those in optical
images. A good example is the San Francisco
Airport (at circle A in left image), which in
radar is quite black but would have various
shades of gray in most Landsat bands. Can you
explain this?
Landsat-TM false colour composite image
ERS-1 radar image
? Wageningen UR 2002
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Question 5 The more obvious difference is the
distortion in the shape of features that have a
strong three-dimension expression, such as
mountains. In Landsat images, the mountains near
San Francisco appear "normal", that is, they have
slopes on either side of the mountain crests that
are similar in slope angles (e.g. at circle B in
left image). But, in the radar image one slope
side seems stretched out and the opposite slope
appears shortened this is a hallmark of radar
imagery known as layover. Can you explain this
layover effect?
Landsat-TM false colour composite image
ERS-1 radar image
? Wageningen UR 2002
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Question 6 One property of radar pulses gave
rise to an extraordinary image acquired from
SIR-A (Space Shuttle Imaging Radar) in November
1981. The color scene to the left is a Landsat
subimage of the Selma Sand Sheet in the Sahara
Desert within northwestern Sudan. Because dry
sand has a low dielectric constant, radar waves
penetrate these small particles several meters
(about 10 ft). The inset radar strip trending
northeast actually images bedrock at that general
depth below the loose alluvial sand and gravel
which acts as though almost invisible. It reveals
a channeled subsurface topography, with valleys
that correlate to specularly reflecting surfaces
and uplands shown as brighter.What frequency
band did SIR-A use? Why is this better suitable
for this type of application than e.g. the ERS-1?
? Wageningen UR 2002