The interpretation of Synthetic Aperture Radar (SAR) satellite images involves the identification and measurement of targets in a radar image. This radar image is the result of extensive data processing, including calibration, geometric, and radiometric corrections.
Analysing SAR images: finding your targets
A target may be any feature or object which can be observed in an image. This may be a point, line or area feature. This interpretation of targets in SAR images is performed manually or digitally. The visual interpretation requires little, if any, specialised equipment, while digital analysis requires specialised, and often expensive equipment and software.
Interpreting radar data can sometimes be a subjective process, meaning that the results will vary with different interpreters. This is why visual and digital analysis of SAR remote sensing imagery are not mutually exclusive and are usually both used in parallel.
Find here an example of how SAR image interpretation teaches us about ground displacement around the Kilauea volcano in Hawaii, USA.
Elements of SAR images
The recognition of a target is key to interpretation and information extraction. Observing the differences between targets and their backgrounds involves comparing different targets based on any, or all, of the visual elements of tone, shape, size, pattern, texture, shadow and association. There are several degrees of complexity, to be considered when analysing SAR imagery, each revealing different levels of detail about the image:
- Primary basic elements of a SAR image are tone and colour.
- Secondary spatial arrangements of tone are size, shape and texture.
- Tertiary spatial arrangements of tone are pattern, height and shadow.
- Higher spatial arrangements of tone are site and association.
So what do these elements and levels of detail mean?
Tone refers to relative brightness or colour of objects in an image, fundamental element for distinguishing between different targets or features. The variations in tone also allows the elements of shape, texture, and pattern of objects to be distinguished.
Size is a function of scale, it is important to assess the size of a target relative to other objects in a scene, as well as the absolute size, to aid in the interpretation of that target. For example, in an area with a number of buildings, large buildings such as factories or warehouses would suggest commercial property, whereas small buildings would indicate residential use.
Shape refers to the general form, structure, or outline of individual objects. For example, straight edge shapes typically represent urban or agricultural (field) targets, while natural features, such as forest edges, are generally more irregular in shape.
Texture refers to the arrangement and frequency of tonal variation in particular areas of an image. Rough textures would consist of a mottled tone where the grey levels change abruptly in a small area, whereas smooth textures would have very little tonal variation. Smooth textures are most often the result of uniform, even surfaces, such as fields or asphalt. Targets with rough surfaces have irregular structure, such as a forest canopy.
Pattern, refers to the spatial arrangement of visibly discernible objects. Typically, an orderly repetition of similar tones and textures will produce a distinctive and ultimately recognizable pattern. For example, cultivated areas and urban areas are good sample of pattern.
Shadow, may provide an idea of the profile and relative height of a target/targets which may make identification easier. Also reduce or eliminate interpretation in their area of influence, since targets within shadows are much less (or not at all) discernible from their surroundings. Is also useful for enhancing or identifying topography and landforms, particularly in radar imagery.
Association, takes into account the relationship between other recognizable objects or features in proximity to the target of interest, to associate with other features may provide information to facilitate identification. For example, commercial properties may be associated with proximity to major transportation routes.