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38. Radar image interpretation

Radar images have certain characteristics that are fundamentally different from images obtained by using optical sensors such as Landsat, SPOT or aerial photography. These specific characteristics are the consequence of the imaging radar technique, and are related to radiometry (speckle, texture or geometry).

During radar image analysis, the interpreter must keep in mind the fact that, even if the image is presented as an analog product on photographic paper, the radar "sees" the scene in a very different way from the human eye or from an optical sensor; the grey levels of the scene are related to the relative strength of the microwave energy backscattered by the landscape elements.

Shadows in radar image are related to the oblique incidence angle of microwave radiation emitted by the radar system and not to geometry of solar illumination. The false visual similarity between the two types of images usually leads to confusion for beginners in interpretation of radar images.

Elements of interpretation of radar imagery can be found in several publications for example, in "The use of Side-Looking Airborne Radar imagery for the production of a land use and vegetation study of Nigeria" (Allen, 1979).

Grey levels in a radar image are related to the microwave backscattering properties of the surface. The intensity of the backscattered signal varies according to roughness, dielectric properties and local slope. Thus the radar signal refers mainly to geometrical properties of the target.

In contrast, measurements in the visible/infrared region use optical sensors where target response is related to colours, chemical composition and temperature.

The following parameters are used during radar imagery interpretation:
-tone
-texture
-shape
-structure
-size

Several principles of photo-interpretation can be used for radar imagery interpretation and we can distinguish three steps:
- photo reading:
this corresponds to boundaries recognition on the basis of the previously listed parameters.
- photo analysis:
this corresponds to the recognition of what is within the boundaries previously identified.
- deductive interpretation of image:
At this stage, the interpreter uses all his thematic knowledge and experience to interpret the data.

Before describing texture, we can propose the following definitions:

-Tone
Radar imagery tone can be defined as the average intensity of the backscattered signal. High intensity returns appear as light tones on a positive image, while low signal returns appear as dark tones on the imagery.

-Shape
It can be defined as spatial form with respect to a relative constant contour or periphery, or more simply the object's outline. Some features (streets, bridges, airports...) can be distinghished by their shape. It should be noted that the shape is as seen by the oblique illumination: slant range distance of the radar.

-Structure
The spatial arrangement of features throughout a region with recurring configuration.

- Size
The size of an object may be used as a qualitative recognition element on radar imagery. The size of known features on the imagery provides a relative evaluation of scale and dimensions of other terrain features.