Orthoimages

Orthoimages, or orthophotos, are georeferenced raster images created by orthorectifying aerial or satellite imagery to remove geometric distortions caused by camera perspective and terrain relief. By integrating sensor geometry with a digital elevation model and, if available, ground control data, each pixel is assigned a precise ground coordinate. The resulting image has a uniform scale across the scene and represents true ground distances, allowing measurements to be made directly as on a map.

The creation process involves several steps: data acquisition, exterior orientation, correction for terrain relief using a digital elevation model, resampling, and often radiometric normalization followed by mosaicking to form seamless coverage. The output is commonly delivered in formats such as GeoTIFF or JPEG2000, with metadata specifying the coordinate reference system, ground sample distance, and coverage area.

Inputs can include drone or UAV imagery, aircraft photography, and satellite imagery, at varying resolutions. Orthoimages are widely used in GIS and mapping applications, including urban planning, land surveying, environmental monitoring, resource management, and disaster response. They provide a reliable base layer for overlaying vector data and performing accurate measurements, and they can be produced as true orthophotos or near-orthophotos depending on DEM quality and processing.

Limitations include occlusion by tall structures, shadows, and vegetation, as well as reliance on the accuracy of the DEM and sensor data. Temporal differences between the imagery and current ground conditions can also affect usefulness. Quality assessment typically reports planimetric accuracy against ground control points, the ground sample distance, and related RMSE metrics.