Radar Imagery Interpretation: Shadowing and Foreshortening

Look Direction

• Opposing look directions can produce different tones on an image due to row direction related to planting, tilling, or harvesting in agricultural fields.
• In areas with high relief, opposing look directions are necessary to fill in areas of radar shadow.
• Fixed-looking systems like RADARSAT can acquire two look directions using ascending (east-looking) and descending (west-looking) passes.

Local Incident Angle

• Local incident angle (θloc) is defined as the angle between the radar line of sight and the line normal to the local slope.
• θi is the flat-earth or ellipsoid incident angle.
• Local incident angle has a large effect on image brightness per pixel.
• Local incident angle is the largest source of error in radiometric calibration.

Local Incident Angle Effects

• Effects of local incident angle include changes in image brightness per pixel.
• Local incident angle affects radiometric calibration.

Topographic/Relief Displacement

• Topographic displacement differs between radar and optical systems due to different imaging geometries.
• Shadowing in radar imagery is important for terrain relief interpretation.
• Radar shadow is a key indicator of terrain relief.

Radar Shadow

• Radar shadow occurs in areas on the ground surface not illuminated by the radar due to geometry and scene relief.
• Radar shadows appear very dark in tone on the imagery.
• Shadows occur in the down-range direction behind tall objects.
• Shadows are a good indicator of radar illumination direction if annotation is missing or incomplete.
• Shadowing becomes more prominent towards far-range due to increasing incident angle.
• Radar shadow can provide information about the scene, such as an object's height.

Foreshortening

• Foreshortening is the appearance of compression of features in the scene that are tilted towards the radar.
• It leads to a relatively brighter appearance of these slopes.
• Foreshortening is at a maximum when a steep slope is orthogonal to the radar beam.
• Effects of foreshortening are reduced with increasing incident angles.
• At the grazing angle, where incident angles approach 90°, foreshortening effects are eliminated, but severe shadowing may occur.

Resolution Cell

• Resolution cell is a function of range resolution (rR) and azimuth resolution (rA).
• Slant range (rR) and ground range (rGR) images have different scales.
• Differences between slant and ground range resolution are highest at small incident angles.
• rR and rGR converge at high incident angles (e.g., 70°).