Nadir

The nadir is the position on the earth’s surface that lies immediately below a flying sensor. In remote sensing, geospatial sciences and applications, the concept of nadir plays an important role in the determination of the geometry of the sensors, viewing angles, distortion, ground coverages, and accuracy of spatial information. If the sensors capture the images directly downwards from their positions, it is referred to as a nadir looking imaging process. The use of nadir imagery is common in aerial imaging, drone surveying, satellite imaging, photogrammetry, GIS, and remote sensing of the earth’s surface. The distance and angle of the sensors from the nadir points may affect the resolution, perspective, and geometric accuracies of the images taken. It is useful when examining orthophoto maps, digital elevation models (DEMs), digital surface models (DSMs), LiDAR surveys, and satellite imagery. An appropriate orientation of the nadir camera during aerial and drone mapping may ensure consistent imagery for the photogrammetric analysis. The term is often contrasted with the zenith.