Remote Sensing: Understanding Foreshortening and Layover

Relief Displacement in Radar Imagery

• Primary factor influencing relief displacement is the local incidence angle (θi).
• When the angle of radar-facing slope is less than θi, displacement decreases and may be negligible.
• At a local incident angle of 90º, terrain is imaged with no relief displacement, appearing true to scale.

Horizontal Displacement Pattern

• Radar sensor experiences horizontal displacement that is dependent on the incident angle and terrain relief.
• When the angle of radar-facing slope equals θi, no further displacement occurs, maintaining a stable radar image.

Terrain Distortion Direction

• Terrain distortion in radar imagery typically occurs in the direction of the radar beam, affecting the appearance of the landscape.

SAR Imagery Distortion

• The distortion caused by the interaction of the radar beam with the terrain is called geometric distortion.

Object Displacement

• Tall objects in radar imagery are represented as displaced towards the radar, often resulting in a pronounced visual effect.

Imaging Radar Perspective

• Imaging radars provide a side-looking perspective, capturing terrain at an angle rather than straight down.

Aerial Photography

• Aerial photography inherently contains vertical relief displacement, affecting spatial accuracy.

Radar Shadowing

• Radar shadowing occurs when an object blocks the radar signal, leading to areas without radar return information.

Effects of Foreshortening

• Foreshortening causes terrain features to appear compressed, differing from their actual size.
• Foreshortening is maximized at oblique incident angles, particularly below 30º.

Incident Angle Trade-off

• Choosing a steeper incident angle enhances ground detail but may increase foreshortening and make interpretation difficult.
• As incident angles increase, foreshortening effects become more pronounced, leading to distorted imagery.

Layover in Radar Processing

• Layover occurs when the height of an object exceeds its distance from the sensor, causing overlapping echoes in radar images.
• Interpreting SAR images with layover presents a significant challenge, as features may appear incorrectly positioned.

Layover and Foreshortening Relationship

• Layover is more prominent at steep incident angles, contributing to the complexity of image interpretation.
• Extreme horizontal displacement in layover can distort the perceived dimensions and locations of objects in an image.