INTERPRETATION OF

MICROWAVE REMOTE

SENSING IMAGES

EXERCISE

dr.ir. Jan Clevers

Centre for Geo-Information

Dept. Environmental Sciences

Wageningen UR

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.

Wageningen UR 2002

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

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

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?

A B

C

Wageningen UR 2002

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

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

A

B

Wageningen UR 2002

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

A

B

Wageningen UR 2002

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?