Lecture 07: Photogrammetry - Stereo Pairs and Stereovision

Summary

Lecture 7 explores stereo pair and stereo vision in photogrammetry, demonstrating how overlapping aerial photographs can be used to create 3D models of terrain. It covers the importance of parallax, triangulation, and stereoscopic fusion for depth perception, both through human vision and specialized devices like stereoscopes. Key concepts include stereo pairs with at least 60% overlap, parallactic angles, and proper base-to-height ratios. The lecture also discusses practical exercises, stereograms, and advanced equipment such as stereometric and 360° cameras. Applications range from terrain mapping and slope analysis to disaster management, offering a comprehensive understanding of 3D visualization in surveying and remote sensing.

Takeaways

• 😀 Stereo vision uses overlapping aerial photographs to create 3D models of terrain.
• 😀 Individual photographs are suitable for 2D mapping, but stereo photogrammetry is required for height and relief analysis.
• 😀 Stereo images must be captured from at least two different camera positions to introduce parallax.
• 😀 The principle of triangulation is used to determine coordinates of points common to overlapping images.
• 😀 A stereo pair consists of two successive photographs with a minimum of 60% forward overlap.
• 😀 Human eyes fuse left and right images in the brain to perceive depth, creating stereo vision.
• 😀 Stereoscopes are devices that aid in correctly aligning stereo images for proper 3D perception.
• 😀 Parallax angle decreases with object distance, and the i-base (distance between eyes) is typically 6–7 cm.
• 😀 Requirements for accurate stereoscopic vision include overlap, similar exposure, same scale, brightness, parallax, and base-to-height ratio of 0.25–2.0.
• 😀 Advanced equipment like stereometric cameras and digital photogrammetry systems allow rapid collection and processing of stereo images for 3D modeling.
• 😀 Stereograms and simple exercises with fingers can help train the human eye to fuse images and perceive depth without equipment.
• 😀 Stereo vision applications include terrain slope analysis, flat area identification, vegetation mapping, landslide monitoring, and panoramic imaging.

Q & A

What is the primary purpose of stereo vision in aerial photogrammetry?

The primary purpose of stereo vision is to create a 3D model from two or more overlapping aerial photographs, allowing for the measurement of terrain height, slope, and relief.

What is a stereo pair and what is the minimum overlap required?

A stereo pair consists of two successive photographs with overlapping areas, and a minimum of 60% forward overlap is required to generate a stereo model.

How does parallax contribute to stereo vision?

Parallax is introduced by capturing images from different camera positions. It creates a relative displacement of objects in the images, which is essential for perceiving depth and constructing 3D models.

Explain the role of the human eye in stereo vision.

The human eyes receive left and right images, which are focused on the retina and fused in the brain. The brain interprets depth, object size, shape, texture, color, and location to perceive a 3D model.

What is the i-base and how does it affect depth perception?

The i-base is the distance between the eyes, typically 6–7 cm. It affects depth perception by determining the angle of convergence (parallactic angle) at which the brain interprets object distances.

What is the parallactic angle and how does it relate to object distance?

The parallactic angle is the angle formed at the eye between rays from object points. Farther objects produce smaller parallactic angles, while closer objects have larger angles, influencing depth perception.

Why is consistent exposure, scale, and brightness important in stereo imaging?

Consistent exposure, scale, and brightness ensure that overlapping photographs can be accurately fused into a stereo model without contrast issues, mismatched scales, or difficulties in depth interpretation.

What is the function of a stereoscope?

A stereoscope aligns the left and right images for each eye to prevent cross-vision, facilitating accurate fusion and perception of the 3D model.

How can stereo images be captured using a single camera?

Using a single camera, two overlapping photographs are taken sequentially from slightly different positions to introduce parallax. Alternatively, a stereometric camera with two fixed cameras can capture both images simultaneously.

What are stereograms and how can they be used without instruments?

Stereograms are pairs of overlapping images arranged closely. By focusing the left image with the left eye and the right image with the right eye, one can perceive a 3D effect without using instruments like a stereoscope.

What are some practical applications of stereo vision in mapping and engineering?

Applications include terrain modeling, measuring slopes and relief, landslide volume estimation, civil engineering project planning, and creating panoramic 3D mosaics using drones or vehicles.

What is the base-to-height ratio and why is it important?

The base-to-height ratio is the ratio of the distance between the camera positions (base) to the flying height of the aircraft (height). It typically ranges from 0.25 to 2.0 and affects the exaggeration of height in the stereo model for better visualization.