orthophotography

Orthophotography refers to aerial or satellite imagery that has been geometrically corrected to remove distortions caused by camera tilt, lens distortion, and terrain relief, so that the image has a uniform scale. An orthophoto represents ground features in their true positions and shapes, enabling direct measurement as if viewed from directly overhead.

The processing step is known as orthorectification. It combines sensor geometry with a digital elevation model (DEM) or digital terrain model, along with ground control information if available, to map image coordinates to precise ground coordinates. The result is an orthophoto in which distances, areas, and angles correspond to real-world values, making it suitable for Geographic Information Systems (GIS) and cartographic work. Orthophotos can be generated from both aerial photographs and satellite imagery, and may be produced at various resolutions and map scales.

Applications of orthophotography are broad. It serves as a base map layer for urban planning, cadastral surveys, infrastructure management, environmental monitoring, agriculture, and disaster response. It also enables accurate measurement of features and overlay with other spatial data sources, such as vector maps and 3D models.

Advantages include a uniform scale across the image, geometrically corrected measurements, and seamless mosaics of large areas. Limitations involve dependence on high-quality elevation data and sensor models, residual distortions in areas with steep relief or dense canopy, potential radiometric differences after processing, and higher cost and processing requirements compared to unrectified imagery.