Offnadir

Off-nadir is a term used in satellite and airborne imaging to describe the angular deviation of the sensor's line of sight from nadir, the point on Earth's surface directly beneath the platform. The off-nadir angle is measured at the target as the angle between the local vertical and the sensor's viewing direction. Sensors are often capable of scanning to modest off-nadir angles to widen their swath or to acquire oblique views, within design limits of the instrument.

Geometric and radiometric consequences: As the off-nadir angle grows, the ground footprint of a pixel becomes larger and more elongated, and ground-coverage geometry departs from a simple nadir projection. This causes geometric distortions such as foreshortening and parallax, which complicate georeferencing and may reduce apparent spatial resolution at the scene edges. The longer atmospheric path length for oblique views can increase atmospheric absorption and scattering, reduce radiometric contrast, and amplify shadows and sun-glint.

Operational considerations: Off-nadir imaging trades spatial resolution for swath width and revisit capability. It can help to view areas obscured by clouds from a different angle, or to acquire stereo pairs for three-dimensional mapping. However, higher off-nadir angles require more sophisticated geometric correction, calibration, and possibly supplemental elevation data to produce accurate products.

Applications and limitations: Off-nadir data are used in Earth observation, mapping, disaster response, and surveillance. For precise mapping and change detection, nadir data or well-calibrated off-nadir products with robust sensor models are preferred. The suitability of off-nadir imagery depends on sensor design, viewing geometry, and the intended use.