Optical acquisition modes

Optical imagery can be captured in a variety of acquisition modes. These modes have varying levels of complexity and can be used for different purposes.

Optical collections offer imagery that is captured in three primary acquisition modes: mono, stereo, and tri-stereo.

Mono or monoscopic acquisition mode is when an imaging system captures a single image of an area of interest (AOI). This is a relatively simple acquisition mode and can be used to obtain imagery for general visualization, mapping, and basic analysis.

Stereo or stereoscopic acquisition mode is when, during the same or subsequent pass, an imaging system captures at least two images of an AOI, with different viewing angles. The different viewpoints will still be in the same orbit. Stereo pairs provide depth information that can be used to produce digital elevation models and derive terrain elevation information.

Tri-stereo acquisition mode is a more complex type of stereo mode, where a third additional image is captured of the AOI in a near-vertical position. Tri-stereo triplets can be used for dense urban or mountainous areas, where elevation may block the line of sight of the sensor to the AOI.

Some geospatial collections use only along-track or across-track scanning, while some systems can switch between these modes. Stereo pairs and tri-stereo triplets almost always use along-track scanners because across-track stereo or tri-stereo images would be separated by one or more orbits.

Along-track scanners use a line of detectors that point at a target. The detectors capture imagery parallel to the platform’s movement.

Along-track scanners have a longer dwell time, which improves radiometric and spatial resolution.

Across-track scanners use a single detector with a rotating mirror. The mirror sweeps the target from side to side, perpendicular to the flight direction. The detector captures imagery at right angles to the platform’s movement.

As across-track sensors use a single detector, they provide a consistent pixel response. This results in more accurate reflectance values.

Mono imagery is available on the console, but the availability of stereo and tri-stereo may differ depending on the collection. Some imagery is only available upon request.

^*Upon request.

What is the optimal B/H ratio?

Base/height ratio refers to how close apart stereo or tri-stereo images are taken relative to the height of the passing imaging system. It’s an important parameter for imagery that is going to be used to generate elevation models.

For automated elevation model generation, the optimum B/H ratio is 0.25. Otherwise, follow these guidelines:

• For flat regions, use a higher B/H ratio.
• For mountainous or dense urban areas, use a lower B/H ratio.

Can I create stereo pairs out of mono imagery?

If single-pass image pairs or triplets aren’t available, mono imagery with similar inclination angles over the same AOI can be used. This will require additional co-registration steps to ensure images align correctly for any subsequent analysis. This approach isn’t suited for AOIs with vegetation cover, and can’t be used for subsequent vegetation management analysis.

Artificial stereo pairs can be ordered using the Python SDK. For more information, see the Searching for real and false stereo Jupyter notebook.

Why do stereo and tri-stereo acquisitions take longer?

Stereo and tri-stereo acquisitions require imaging systems to reorient their sensors in real time to capture images from different angles. Executing these maneuvers may not be successful on the first attempt. As a result, these types of acquisitions can take longer than a simpler mono acquisition.

• Stereo data ordering 101
• Stereo imagery, DSM, and machine learning for vegetation management
• Utility vegetation management